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Recently, a new type of a parametrically excited power converter which is made by two C-cores rotated at 90° was proposed and called as Paraformer [1]. The novel characteristic of this device has been remarked as one of the important phenomena which permit to expand the new region of nonlinear magnetics [2], although the operation principles are not yet clearly understood. It is concluded from the present work that the performance characteristics of the bridge-core circuit which we designed are completely agreed with that of the Paraformer, and a novel characteristic of the Paraformer is generated by the bridge circuit. By using the symmetrical or the asymmetrical bridge-core circuit, it is possible to make not only a practical high efficient and high sensitive voltage regulator but also a power converter which can change the output frequency. Furthermore, a large capacity power converter can be achieved by constructing a bridge circuit with 3-5 separate reactors.

A restraint and movement detection device for use in behavioral studies with small animals is described. The apparatus consists essentially of a hinged 2-piece circular neck collar attached to a platform stabilimeter, which records motion by means of a sensitive d.c. bridge circuit designed around strain gauges mounted on flexible supports. An optional tail clamp further restricts body movements. The device is simple, inexpensive, and easy to use.

A hot wire system has been developed which is suitable for operation in liquid mercury at speeds from 1 to 12 cm/sec. The wire was 38 μ diam tungsten with 2.5 μ thick enamel coating, used with 2 A heating current in constant current operation. The Nusselt number was determined experimentally as a function of Peclet number in the range 0

The paper deals with the theory and design of the non-linear Wheat-stone bridge circuits used in voltage stabilizers. It is restricted to the use of the bridge as a source of voltage proportional to the change in the supply voltage and to non-linear elements obeying the law V = kIn, where n and k are constants. Four circuits are considered, and formulae are deduced for the general case of a resistive load across the output terminals. The effect of changes in the various parameters is discussed. Formulae are also given for the load required for maximum power output from the bridge for a given change in input voltage. The paper concludes with a review of the advantages and disadvantages of the various circuits.

Through a specialized capacitor we can achieve a device capable of being employed by commercial paper mills to measure and control the amount of moisture contained in a moving sheet of paper. By such control a mill can achieve dollar savings by producing more paper to exact specifications, and with less down time from paper breaks. Continuous monitoring of the moisture content of a fast moving web of paper can be made by using a spray or fringe field capacitor and a compatible electronic system. The basic principle involved is that the area and spacing factors of the capacitor have been held rigidly constant so that one variable is the dielectric. This dielectric is a web of paper that travels across a stationary capacitance measuring device. As the paper varies from a relatively dry to a wet condition the basic capacitance will change as the dielectric changes. If this varying capacitor is connected into a special voltage bridge circuit and the bridge is excited with 5000 cycles, the use of an amplifier, a 5000 cycle phase detector, and a mechanical feedback loop to a variable capacitor will rebalance the bridge to a null condition. With the rebalanced variable capacitor position connected to a pen the per cent moisture deviation of a web of paper can be read on a chart.

The cryo-orienter is used for neutron diffraction studies of single crystals between liquid helium and room temperatures. It combines a double Dewar cryostat with a three-circle goniostat. The temperature of the crystal is controlled by a combination of exchange gas and a bridge circuit to a stability of ±0.01 Kelvin degree. A ring assembly allows the cryostat to be angularly displaced ±50° from vertical (χ), and to be rotated 360° (ϕ), which provides crystal motion about two axes with ±0.01° precision. The ring assembly can be rotated about the diffraction vector. Crystals can be mounted along a non-major zone and sufficient data can then be collected with one crystal mounting. The ARCADE computer system controls the entire data collection.

A general purpose instrument is described which is capable of providing temperature control to better than 1 mK for long periods in a wide variety of cryogenic applications. The main first and second order effects contributing to temperature instability are taken into consideration. For a range from 2 to 300 K at least two temperature sensing elements are required. The bridge circuit described is designed for use with an Allen-Bradley type carbon thermometer below 20 K and a copper or platinum thermometer above 20 K. Details of the circuit and constructional features of the controller are included.

Much information about the density of electron states in a superconductor may be obtained using electron tunneling techniques. It is of particular interest to measure the normalized dynamic conductance of superconducting tunnel junctions as a function of applied voltage. This quantity is the ratio of the dynamic conductance (di/dv)s when one or both metallic members of the tunnel junction are in the superconducting state, to the dynamic conductance (di/dv)n when both members are normal. A new method has been developed which enables measurements to be made of (di/dv)s/(di/dv)n to a few parts in ten thousand. With this method only a bridge circuit, a galvanometer amplifier, and an oscilloscope are used. The galvanometer amplifier has a passband from dc to a few cps and an input noise voltage of about 3×10−8 rms V. The circuits of the bridge and amplifier are presented and analyzed. The operation of the circuits for measuring the characteristics of low resistance specimens (≲500Ω) is described.

A method has been devised for the measurement of the dielectric constant and the specific conductance of aqueous solutions undergoing streaming with velocity gradients between 0 and 10 000 sec−1. The solution to be investigated is placed in the annular space between two concentric cylinders forming the measuring cell. Electrodes are inserted in the walls of the cylinders of which the outer is fixed and the inner is rotated. Two types of cells are used; the electrical field in one case being perpendicular to and in the other case parallel with the stream lines. The impedance of the cell with the solution investigated is determined with a specially constructed Schering bridge making measurements possible in the frequency range 0.05 to 5 megacycles per second and at specific conductances of 1 to 200 μmho per centimeter. The advantage of the bridge circuit employed is that the conductance of the cell is balanced by a variable air condenser, making possible an accuracy of 0.1 percent in the values for both conductance and dielectric constant. Excellent mechanical and electrical stability and a low interference level are necessary for high accuracy. Experiments with solutions of sodium thymonucleate show that the current interpretation of the dielectric properties of macromolecules must be revised. The term streaming dielectric investigations is suggested as a suitable name for studies with the type of instrument described.

A design of a solid state diode magnetic fluxmeter utilizing the effect of magnetic Field on its forward current characteristic is reported. Using three suitable diodes, a bridge circuit is constructed for the automatic compensation of the effect of tomperature on the sensitivity of the fluxmeter. A typical fluxmeter constructed in the laboratory is able to measure directly a magnetic field from 2 to 19 kilogauss and its transient response is found to bo much better than those of conventional fluxmetera.

Measuring circuits similar to that described above are the most suitable for use in automatic digital bridges with a wide measuring range and in comparator instruments supplied from the mains and fitted with a null-indicator.

A simple null‐indicating saturable‐core magnetometer has been designed as a completely self‐contained unit requiring no associated equipment other than a VTVM or oscilloscope for monitoring the output. The device is useful for detecting static magnetic fields of the order of 0.1 millioersted. It was built to enable the establishment of a near zero magnetic field over a limited volume by means of pairs of Helmholtz coils. A Hartshorn bridge circuit is employed to cancel out the fundamental of the driving frequency in the output portion of the circuit. A test signal incorporated into the instrument affords a check on the sensitivity.

The fundamental principles of electric field measurement by the use of inductors and vacuum tube amplifiers are reviewed. An electric field meter is described that employs a rotating inductor and a synchronous ac generator to provide a sign responsive meter reading proportional to the electric field. The synchronized circuits eliminate moving contact rectifiers and promote calibration stability.A simplified field meter using rectified inductor currents to operate a high‐impedance push‐pull bridge circuit is also described. Design features are incorporated in both instruments that permit them to operate continuously under unusually severe storm conditions.

Electrical resistance strain gauges are included in two arms of an a.c. bridge circuit, the off-balance voltage from which thus varies with the strain of the gauges. A change in resistance of the gauges of 1 part in 2000 produces an off-balance voltage from the bridge of 3 mV. giving full scale deflexion on the 4 cm. diameter cathode-ray tube screen. In the twelve-channel system described, the off-balance voltage is amplified, rectified and the spot deflexion on the twelve cathode-ray tubes continuously recorded by one camera using 5½ in. wide film. The deflexions are measured directly from the film with a travelling microscope. The frequency of the bridge supply voltage is 500 c/s enabling transient strains of duration 0.1 sec. to be recorded.

Drift of the base line caused by imperfect control of temperature or voltage of a gas-chromatographic catharometer may be eliminated by including one shunt and one series resistance with the catharometer hot elements in the Wheatstone's bridge circuit. Equations enabling the positions and values of the resistances to be calculated from the observed drift are given. Experience shows that use of the compensating resistances enables a catharometer to be used more sensitively without the trouble and expense of careful temperature and voltage control.

By means of a DC bridge circuit one microelectrode was used for simultaneously passing current and recording transmembrane potentials. In some cells, depolarization increased the frequency of discharge whereas hyperpolarization decreased the frequency; the frequency/current relation was sigmoid. In other cells, polarizing currents were without effect upon frequency. The change in action potential magnitude was in proportion to the degree of polarization. From control values of about 5 mv/sec., the slope of the pacemaker potential increased to 60 mv/sec. upon depolarization and diminished to zero upon hyperpolarization. In many cells a transient hyperpolarization was produced on the cessation of depolarizing currents. The voltage/current relationship was linear and had a slope of about 13 M ohm. With an AC bridge circuit, the cell capacitance averaged 800 pf and the time constant, 9.6 msec. R(m) was estimated to be 480 ohm-cm(2) and C(m), 20 microf/cm(2). The magnitudes of some prepotentials were affected by polarizing currents, which suggests that the prepotentials represent postsynaptic potentials.

Several related methods for determining the calibration formula of a continuously scanning bridge circuit using on‐line data are analyzed and discussed. High‐resolution resistance thermometry requires a sensitive, accurate conversion of off‐balance voltage measurements to resistance measurements. The objective is to properly determine the relation R = f(V,RB,Ci), where V is the bridge output‐voltage, RB is a balance resistance, and the Ci are unknown coefficients that depend on RB and other fixed circuit parameters. Linearization to the form R = RB + C1(RB)V can yield a fairly accurate conversion from V(t) to R(t). If Ṙ (t) or higher derivatives are required, linearization leads to systematic errors and a more sophisticated method is needed. From the analysis of three techniques for determining corrective terms, we conclude that parameterization of a model for the bridge circuit is the most satisfactory procedure. An empirical, self‐consistent method is useful if the circuit model is unknown or extremely ...

A method is described for measuring both components of the complex susceptibility, χ = χ' - j χ″ by means of a twin T bridge circuit. The results of some measurements in parallel static fields for a DPPH sample are given. The relaxation time τ is found to depend on the external static field.

A radio‐frequency signal, gated at an audio rate, is applied to a bridge circuit. One arm of the bridge circuit is a phototube or a photomultiplier; the corresponding arm is a capacitor. The bridge is balanced in the absence of photocurrent, unbalanced in its presence, with the degree of unbalance being proportional to the average value of the photocurrent. (Dark current is canceled by an auxiliary circuit.) The null detector is an ac amplifier having a bandpass centered at the audio gating frequency, followed by a synchronous detector tuned to the same frequency. The circuits require no critical adjustments; impedances are well below the levels used in electrometer circuits, and the ultimate sensitivity is limited only by the statistical fluctuations in the dark current. At room temperature, the stability of the zero level is determined by the constancy of the mean level of the dark current. The new method makes it possible to produce stable instruments with a maximum full‐scale sensitivity of 4×10−9 lu ...

A new form of microwave duplexer, capable of handling twice as much power as an equivalent balanced duplexer is described. It consists of a microwave bridge circuit and a power-sensitive half-wavelength phase shifter. A simple gas-discharge tube is used in the phase shifter, which changes the length in one arm of the bridge circuit by a half-wavelength for high-power and low-power microwave pulses, respectively. The performance of one form of phase-shift duplexer has been measured over a frequency range from 8500 mc to 10,000 mc. The vswr is less than 1:2 and the receiver isolation is greater than 30 db over most of the waveband. This is comparable to the performance of a balanced duplexer using the same components. The power handling capacity of the phase-shift duplexer is intrinsically twice as great as that of the balanced duplexer. For example, at a wavelength of 3 cm the phase-shift duplexer will operate unpressurized at a peak power level of 200 kw with a 2:1 safety factor on breakdown, whereas performance of a balanced duplexer at this power level is marginal.

Measurements are reported on the Hall coefficient of polycrystalline samples of pure Cu and Ag and of some of their diluted alloys, in particular Ag-Mn (0.01–4 at.%). It was possible to attain a high sensitivity of 10-10 V by using an alternating current of low frequency in a bridge circuit. For the pure metals the Hall coefficients are found to be field dependent with great variations from sample to sample. This may perhaps be connected with differences in texture of the polycrystalline samples. In the diamagnetic alloys Ag-Au and Cu-Sn no field dependence was found. The Hall voltage of the Ag-Mn alloys can generally be considered as the sumof two terms, one of which has a normal character, while the second one is proportional to the magnetization due to the manganese content. For the most diluted alloys, however, such a simple description does not apply.

1. A new device was developed for measuring relative light intensity in plant communitites by means of a pair of CdS photoconductive cells as the light detector. The cells were mounted in metal tubings of 12 mm diameter to avoid disturbing the natural structure of a sward. 2. The principle of the measurement was based on the character of CdS cells that the logarithmic value of the electric resistance vs. that of the light intensity relation is linear under some range of light value. When cells having the same character were placed under two different light intensities the ratio of electric resistance between them corresponds with the ratio of the given two light intensities. 3. If the CdS cells A and B in a bridge circuit (fig. 1-a) were placed under full sunlight and transmitted light respectively, the ratio of the resistance of the cells A : B could be converted to the known ratio of C : D when the bridge was ballanced by the μA meter. The dial of the variable resistor C can be calibrated to the relative light value of B to the full sunlight. The calibration was performed by a series of glass filters having known neutral densities. 4. A slight curvature of the light-resistance relation of the cell was found in the higher range of the light intensity (fig. 2 & 3) and it caused a bias of the measurement under changeable sunlight conditions. The bias was of such a nature that it could be cancelled by a compensating circuit including the third CdS cell D2 placed under full sunlight and attached to the side D as shown in fig. 1-b. The cell D2 reduces the resistance of the side D in response to the increasing light intensity and the compensation is properly regulated automatically so as to accord with the present relative light value by a variable resistor D1 ganged with the measuring dial C. 5. Same results obtained using the device are presented in fig. 6 with respect to the changes in ground level relative light value accompanied by the leaf area indices of various swards. Sampling errors in the case of ladino clover and orchardgrass swards are discussed. It was found that ca. 50 readings are appropriate for sampling and their logarithmic values should be averaged in order to estimate the true mean value within ± 0 06 (ca. 5 % of the length of logarithmic scale 2-0 or 100-1 in percentages) with 95 % confidence.

This Part of the paper sets out the agreed specification of the vision transmitter of the London Television Station. Details of the valves available for meeting the requirements are given, and their calculated performance in the balanced bridge circuit of the final modulated amplifier is outlined. This is followed by an examination of:? (1) The mechanical assembly, with particular reference to the essential features governing the design of the various components. (2) The control of the amplifier by the television signal, and the conditions relating grid input-volts and outputpower to the aerial feeder. (3) Special requirements of the system. The Appendix includes a note on the determination of the compensating capacitance for grid-lead reactance, and general data regarding the transmitter.

A condenser-type soil strain meter for measuring local strains under both static and dynamic loading conditions is described. The strain meter consists of a bridge circuit connecting four condensers, one of which is the measuring condenser. The measuring condenser is formed by two needles embedded into the soil at a certain distance. When the soil is strained, spacing of the condenser changes and, consequently, a potential difference will occur at the bridge and can be recorded by means of an oscillograph. The authors have demonstrated that there is no need to measure the initial spacing between the two needles, provided the linear relation between the soil strain and the change in voltage is accurately obtained from calibration tests. Two applications of the strain meter, i.e., one-dimensional stress wave propagation test and unconfined compression test, were carried out using the device to measure the strains in the soil samples. The test results have shown that the condenser-type soil strain meter can be satisfactorily used to measure the one-dimensional normal strain in the soil samples and it is particularly suitable to measure dynamic strains in soil. /AUTHOR/

This paper describes a type of instability or triggering observed in the half-wave bridge circuit. The circuit, described in the literature by Lufcy and others, is shown schematically in Fig. 1(A).2?5 An explanation of this triggering is presented based upon a type of reverse-slope magnetic-loop operation not previously described in the magnetic-amplifier literature (see acknowledgment). Figs. 2(A) and (B) are two commonly used models of the dynamic loops of ?square-loop? magnetic-core materials. The paper shows that neither model predicts the observed triggering but that a loop having the ?reverse slope? of Fig. 2(C) readily predicts the instability. In Fig. 2(C) the lower minor loop lies to the right of upper minor loop, and lines connecting their centers or flanks would have a negative or reverse slope. The reality of this reverse-slope phenomenon has been determined experimentally, and verifying data are presented. The reader is cautioned that the reverse-slope phenomenon applies only to minor hysteresis loops and not to major loops operating over the entire range from positive to negative saturation. The reader should also be aware that no model of minor- or major-loop operation can be expected to apply to other than a limited range of exciting conditions.

1) An apparatus for the study of gas metabolism in small laboratory animals has been described. This apparatus has the advantages of keeping normal physiological conditions at least several hours to the test animal and determining of both oxygen consumed and carbon dioxide at any desired interval.2) The new method of automatic recording with the high-frequency bridge circuit is suggested.3) An example of the results obtained with the apparatus is also shown.

The important features of the transducer bridge circuit considered here are that under certain conditions the circuit output can be made relatively independent of (1) changes in the carrier period, (2) changes in branch resistance, and (3) changes in capacitances due to temperature. Additional features are (4) the circuit is located at the transducer and (5) requires only two connecting leads to the necessary energizing and indicating equipment, and (6) the branch containing the transducer can be balanced by a pure resistor in the other branch. Each of these features is important for integrated circuit considerations. Some pressure transducer assemblies incorporating a hybrid integrated circuit version of the circuit bridge are shown in addition to sample calibrations.

The pole-zero frequencies and phase deviations of four-, six-, and eight-pole broad-band 90° phase-difference networks are derived by algebraic means. The results agree with those obtained by elliptic functions but lead to simpler computer solutions. An RC design method based on a bridge circuit is developed. The conditions for minimum loss, maximum stability, and minimum number of circuit elements are stated for unterminated and terminated network pairs. Computer methods for obtaining network component values and circuit characteristics are described. A timesharing computer program is available for four-, six-, and eightpole networks and all frequency ranges.

The acceptable zeros of the potential of the electrical fields produced by certain dipole distributions in homogeneous volume conductors is discussed. A bridge circuit is described by which a solution of the three-arm and the four-arm central terminals of Wilson may be solved for a zero of potential of the field produced by an arbitrary distribution of dipoles in a homogeneous volume conductor. An acceptable zero of potential for evaluation of the potentials in a locus on the "body" surface distant from the heart is described.

Amplification of a-c signals with magnetic amplifiers is generally more difficult than d-camplification. The conventional full-wave circuits, such as the doubler circuit and the bridge circuit, can yield high performance only by demodulating a-c signals and amplifying the resultant direct current. These circuits remain, in effect, d-c amplifiers and suffer from several shortcomings. For example, the sensitivity is limited by the forward voltage drops of the demodulating elements, the output is affected by d-c drifts of the control characteristics, and the component duplication and ineffeciencies of push-pull circuitry must be used to obtain phase sensitivity. The work of Ramey has led to the development in recent years of a number of high-speed magnetic amplifiers of the half-wave type. These circuits can amplify a-c signals without demodulation, but they also have certain shortcomings. The power gain per stage is generally less than the gain of full-wave circuits, the amplifiers remain subject to the effect of zero drift caused by unbalance of component characteristics, and low-level sensitivity is particularly difficult to obtain. Geyger has shown that the effect of amplifier zero drift on a-c servomotors (or other frequency discriminating output elements) can be eliminated by operating half-wave circuits at a carrier frequency several times the signal frequency.1 None of these circuits, though, entirely achieves the advantages relative to d-c amplification techniques which are associated with true a-c amplifiers.

The instrument described in this paper was designed for the measurement of relative humidity in the slip stream of flying aircraft. As the temperatures to be measured are low, usually between 0°and − 10 °C., the temperature difference between wet and dry bulb thermometers is sufficiently small that, if mercury thermometers are used, in order to obtain the accuracy required, they would have to be of such fine bore that they could not be read from any distance. As a result wet and dry bulb resistance thermometers have been built connected to a special bridge circuit by means of which the temperature difference can be measured to 0.1 °C. or better. The actual dry bulb temperature can be read by a change in the circuit or by a mercury or alcohol thermometer of fairly large bore which can be read at some distance. In the bridge circuit the two thermometers form two arms of a bridge so that temperature variations in the leads are automatically compensated. A slide wire forms a part of the bridge circuit and the constants are so arranged that a very simple relation gives the difference in temperature of the wet and dry bulbs.

This paper describes an instrument suitable for measuring forces over a range from 10 to 12 000 mg weight with a frequency response which is substantially linear up to 100 c/s. The design of the instrument is such that the force is applied to the centre point of a diaphragm. This diaphragm forms one plate of a parallel plate condenser which is in one arm of a high-frequency bridge circuit. The off-balance voltage produced by the variation in capacity of the condenser is amplified, rectified and displayed on a cathode ray tube. Some details of the performance of the instrument are given in the final section.

An analytical balance has been modified by adding a solenoid and magnet so that changes in weight can be balanced electrically. The operation is made automatic by reflecting a beam of light from the balance beam into a dual phototube in a bridge circuit and amplifying the output of the bridge to provide current in the solenoid. Recording the voltage drop across a resistor in series with the solenoid gives a linear weight scale which can be made to read directly in milligrams. An electronic damping circuit enables the balance to follow very rapid changes in weight without oscillating and with an accuracy better than 0.5 percent of full scale for ranges of 100 mg and over.

A new piece of equipment is described for measuring χ′/χ 0 and χ″/χ 0 as a function of field (0 to 4250 Oe), temperature (1.2°K to room temperature) and frequency (200 Hz to 1 MHz). It is about ten times more sensitive than the Hartshorn bridge used in Leiden2) and it is more convenient to operate as the measuring procedure is automatic after initial adjustments have been made. The main component is a bridge circuit of four inductors, built as closely similar to each other as possible. The output from the bridge goes to two phase sensitive detectors which monitor the two outputs, one inphase, one π/2 out of phase, of the bridge. A heterodyne system is used where the input signal to the bridge is obtained by mixing the output from a variable frequency oscillator with that from a 1.5 MHz oscillator and taking the difference frequency, which is phase locked to a master oscillator. The output from the bridge, after preamplification, is mixed with a second output from the variable oscillator and the difference taken again. This gives a 1.5 MHz signal modulated by the magnetic effects in the bridge which is used in the two phase-sensitive detectors. Their output is recorded on an x−y writer. The bridge needs only be balanced to an output of about 50 mV because the sample is moved between two coils and the difference voltage is measured. The use of the same equipment to measure relaxation times longer than 100 ms is also described.

A bridge circuit has been developed, using a Zener diode, which enables measurement of small changes in current or voltage from a preselected value. The circuit features increased sensitivity over a meter whose range includes the total current and is more easily used than a balanced bridge or potentiometer. The design equations are developed and the design procedure outlined for both the current and voltage difference measuring bridges, while a sample calculation is made for the current change measuring circuit. A bridge similar to that in the sample calculation was built and tested. The test circuit displayed less than 1 per cent nonlinearity of meter current vs change of input current for the design region of 460±5 ma. This shift was attributed to a slight heating of the Zener diode or a resistor at the higher test currents, but since the diode used in the test circuit had a smaller power rating than the type originally intended for use, it is not considered to be a serious limitation.

Generally the ohmic potential drop in a galvanic cell that occurs if a rectangular pulse is led through the cell, is compensated by means of a well-known bridge circuit. A better method making use of a phase reverter is described and its features are discussed. Exchange current densities up to 1200 mA/cm 2 can be studied. The error arising from mis-compensation of the ohmic drop does not exceed 5 % in that case.

The electrotonic spread of current was determined by double microelectrodes cemented at various distances. A bridge circuit enabled transmembrane potentials to be recorded at zero distance from injected current. At interelectrode distances of 100 µ or greater, with the electrodes oriented parallel to the cell axes, current flow of 20 x 10–9 amp through one intracellular electrode produced almost no change in potential at the second electrode. At interelectrode distances of 40 µ or less, similar currents through a given electrode pair produced either a substantial or a small change in potential depending on the impalement. With transverse electrode orientation, no substantial interactions occurred at distances of 17 µ or greater. Various criteria, including the congruous decline in resting potential on both channels, indicated when both electrodes impaled the same cell. The degree of electrotonic spread did not depend on the interelectrode distance per se at close distances. Thus, current flow through one cell does not have substantial effect on the transmembrane potentials of adjacent cells.

In this method a capacitance is compared with a mutual inductance in a 1000‐cycle a.c. network. The secondary of the mutual inductance is in the telephone bridge circuit and carries zero current. By using a Wagner ground circuit there is complete silence in the telephone at the balance point. When used as a substitution method two nearly equal condensers can be compared with each other with an accuracy that is much greater than that of the resistances employed. The final balance is obtained by adjusting a small variable self‐inductance in series with the primary of the mutual inductance. For a given capacitance and frequency the value of this self‐inductance is directly proportional to the power factor of the condenser, and the scale of this inductance can be calibrated to read directly the value of the power factor. Vector diagrams are given to show the relations among the capacitance, power factor, and the inductances, and a few typical measurements are recorded.

In order to evaluate potential and current criteria for the cathodic protection of bare low-carbon steel in a high-resistivity environment, specimens were exposed in the laboratory for a period of two months to a soil having a resistivity of about 20,000 ohm-centimeter. Previous work in low-resistivity environments has shown that corrosion can be reduced to a negligible degree by polarizing a steel structure to —0.85 volt (protective potential) with reference to a copper-copper sulfate electrode. In such studies, cathodic polarization curves have also been shown to be useful in indicating the current density required for cathodic protection. In the present study the above criteria were again evaluated. In addition to protecting the steel at the protective potential (free of IR drop), the effect on protection of including IR drop caused by the protective current was also noted. Also, cathodic polarization curves were obtained on a recorder in conjunction with a bridge circuit to eliminate the IR drop. The results show that the best degree of protection was achieved on the specimen controlled at —0.77 volt (without IR) with reference to a saturated calomel half-cell. This is approximately equivalent to the protective potential —0.85 volt with reference to the copper-copper sulfate electrode. Applied current indicated by the break (change-in-slope) in the cathodic polarization curve agreed reasonably well with the actual current necessary to maintain polarization at —0.77 volt (free of IR). The current required for protection was about three times the magnitude of the corrosion current; therefore, the corrosion reaction was either under anodic control (unlike previous studies) or an equivalent type of control which was caused by high resistance at anodic areas. 5.2.4

A bridge circuit is described in which the inductance of a coil is compared to resistances and a capacitance. A brief comparison is made to other similar circuits.

In this paper is discussed the problem of measurement of the small, but not always negligible, phase angles of resistors, with special reference to shielded resistors of 1000 ohms and above. Several types of shielded resistors are described briefly, among which is the ideally shielded, low‐phase‐angle resistor, recently constructed by the writer. The substitution method developed is one in which the resistor, the phase angle of which is desired, is compared, by means of bridge circuit, to a reference standard of a simple geometric configuration and of approximately the same resistance as that of the resistor. The results of measurements, at frequencies of 500 and 60 cycles, on three ideally shielded resistors are given. By a series connection of resistors it is possible to extend the range of the method to 50,000 ohms.