A quick and sensitive method of measuring condenser losses at radio frequencies

The method is essentially a substitution method in which an air condenser is substituted for the condenser being measured, the current being brought to the same value in the two cases by inserting a resistance in series. The success of the method largely depends on the construction of a suitable continuously variable resistance, of which a description is given. With careful use the method will give the value of the difference of the equivalent series resistance between the standard condenser and that under test to within 0.01 ohm, or even less when the circuit conditions are very good. A number of results of condenser measurements are given, and it is shown that the effective series resistance R of the variable air condensers measured can, within the limits of experimental error, be represented by the equation R = r + 1/ aC 2 f where f is the frequency, C the capacity, and r and a are constants of the condenser. r appears to be the resistance due to bad contacts and leads inside the condenser. Results tend to show that with aluminium plates the constant r is liable to be unexpectedly large, although in many cases it is very small. Also with ebonite the value of a appears to decrease with humidity. The advantages claimed for the method are its rapidity, facility of working, possibility of working down to a wave-length of at least 50 metres, its comparatively high sensitivity, and most important, its freedom from errors due to reaction back on to the source.