A method of measuring the effective resistance of a condenser at radio frequencies, and of measuring the resistance of long straight wires

When the effective resistance of a high frequency circuit is measured by a resonance method, it is usual to find that the resistance exceeds the calculated resistance of the coil. Some of the discrepancy may be due to energy loss in the condenser, and it is desirable to have some means of measuring this loss. The energy loss in high power condensers is now measured regularly by thermal methods and may be as small as 0·025 per cent. of the volt ampere product. But a thermal method is impracticable for the small condensers used in a laboratory because the power absorbed would be less than 0·1 W. Most of the energy loss in an air condenser is presumably due to the dielectric supporting the plates and to poor contacts between the plates. Dye has developed ma very elegant method for measuring the energy loss in a condenser, which presumes that all the loss occurs in the solid dielectric. In his method there is a special condenser which consists of two capacities in parallel, and screened from one another. One portion of this compound condenser contains the insulating supports for the second portion. Accordingly the second portion contains no solid dielectric and is a pure air condenser and is presumed to have no loss. This condenser consists of a single circular disc, which may have one of three sizes, contained within a cylindrical box; the plate hangs from a metal stem which is supported on quartz blocks contained in a chamber above the cylindrical box. The total capacity may be considered to be in two parts. One between the metal stem and the case and having a dielectric which is partly quartz and therefore imperfect; the other between the circular disc and the case and having no dielectric except air and therefore being perfect. The condenser to be tested can be connected in parallel with the special condenser and its capacity is adjusted to be equal to that between the circular disc and the case. The disc can be detached from the stem and so leave only the imperfect portion of the special condenser. The condenser under test is then placed in parallel with the imperfect portion, resulting in a total capacity unchanged by the substitution process. But the substitution has replaced a capacity without loss for an equal capacity with loss. The total circuit resistance is measured by a resonance method before and after the substitution and the difference of value is ascribed to the loss in the condenser under test. Since the special condenser is provided with three different discs the resistance of the condenser under test could be measured at three different settings.