L H Ford

The performance of dried and sealed mica capacitors

An excellent stability has been achieved in a mica capacitor by drying it for a year and then mounting it in a sealed container; the capacitance and power factor of this capacitor and others have been measured between 10 and 10 000 c/s for various conditions and are closely correlated.

A note on the calibration of decade condensers

The note describes an application of the Schering capacitance bridge to the calibration of decade condensers for capacitance and power factor.

The effect of temperature on the stability of mica standards of capacitance

Some types of standard mica capacitors of clamped construction show changes of capacitance which approximately obey an exponential law with a characteristic time constant of the order of 10 years. The paper shows that these secular changes are much hastened by raising the temperature 10 degc above the normal ambient, and that the ultimate secular stability is thereby improved. Appreciable transitory effects are caused by changes of temperature; experiments have shown that such changes must be kept both small and slow if the best stability is to be obtained.

The effect of humidity on the stability of inductance standards

The N.P.L. sub-standard inductance coils have been found to vary in value with atmospheric humidity. The changes in inductance for a 10% increase in relative humidity range from +0.7 part in 104 on a 100 μH coil to -0.25 part in 104 on a 10000 μH coil. The changes may be partly explained by dimensional changes in the formers of the coils.

The stability of mica standards of capacitance

The stability of the National Physical Laboratory substandard mica capacitors since their manufacture in 1932 is described, with special attention to the period 1944-1950. The best section, of 0.1 μF, has remained within ± 1 part in 10 000 of its nominal value on capacitance throughout, and its power factor has shown barely detectable changes over the period 1944-1950. The worst section, of 0.01 μF, has changed in capacitance by 1 part in 1 000 since 1936. Seasonal changes, which are due to atmospheric humidity, have been found in both capacitance and power factor on the smaller capacitors; this effect has been confirmed by tests on a 0.01 μF capacitor in dry and wet atmospheres.

The Stability of Inductance Standards

Information about the stability of inductance standards that has been acquired at the National Physical Laboratory during the last thirty years or so is reviewed, and it is shown that the present working standard of mutual inductance, which is of the oil-immersed type and was constructed in 1937, has remained constant within ±2 parts in 100,000 throughout the twelve years it has been in use. The stability of the primary standard is considerably better, but some standards of the ordinary construction have changed by a few parts in 10,000 in a few years. Data are also given for standards of self-inductance.

A Note on the Measurement of Four-Terminal Inductances by Astbury's Method

Astburys method of measuring four-terminal inductances involves the use of a constant inductance resistance in one arm of the bridge. This introduces an indeterminate capacitance to earth at a mid-point of the arm, and renders the method unsuitable for measurements above audio frequencies. If the fixed resistance ratio arms of the bridge are replaced by a Kelvin-Varley potential divider of known phase defect the constant inductance resistance is no longer necessary, and the bridge can be used at frequencies up to 100 kc/s. Experimental results on a 10 ?H. coil show good agreement at all frequencies from 1 to 100 kc/s with the values obtained by other methods.

Note on the use of the Lloyd-Fisher square for permeability measurements

An examination is made of the errors involved in the measurement of normal and incremental permeability on strips assembled in the Lloyd-Fisher square, as used for total loss testing. It is shown that in normal permeability measurements in the range from B = 10,000 to 15,000 gauss, the results obtained by treating the assembled square as a ring give only a lower limiting value for the permeability. In incremental permeability measurements with flux density variations, ΔB, less than 1000 gauss, on the other hand, the errors are smaller and in many cases are not serious in view of the variability of the properties of the material and the large time effects occurring in that range of flux density.

The a.c. properties of resistors and potential dividers at power and audio frequencies, and their measurement

The alternating-current properties of resistors are briefly discussed, and the time constant and equivalent series inductance and shunt capacitance are defined. The reasons for the variations with frequency are outlined. The determination of the properties is based on standard resistors with windings of simple geometry, for which the inductance and capacitance are calculable. The paper describes methods for comparing such standards with ordinary resistors, for values ranging from a small fraction of an ohm up to about a megohm; particular attention is given to potential dividers of the Kelvin-Varley type, which have great utility in these measurements. The choice of method depends upon the value of the resistor and, for low values (less than, say, 10 Ω), whether the resistor is a two- or four-terminal one.

A note on the measurement of capacitance by ballistic methods

The note describes a simple and convenient method by which a capacitance can be measured ballistically in terms of a mutual inductance, which can be either fixed or adjustable. The constants of the galvanometer are eliminated from the equations, and the accuracy claimed is 0.1 per cent. The normal range, which is readily extended, is from 0.1 to 1 μF.