Abstract :
This paper describes a simple way of making a sub-standard resistor, of value about one ohm, suitable for use at frequencies up to about 10 Mc/s and having substantial current-carrying capacity. In essence it is a revival of bifilar flat-strip construction in contrast to a bifilar of round wire, a method which was commercially used at low frequencies but did not achieve popularity. A bifilar strip of Eureka, 2 mils thick, is separated by a thin mica leaf and clamped tightly between clamping boards of brass or Keramot. It is shown that the time-constant of the assembly depends only on the thickness of the strips and of the mica leaf. The calculated time-constant is 10¿8 sec. The resistor was developed primarily for the purpose of obtaining an oscillogram of pulses of current which were substantially rectangular in shape and lasting about 15 microsec, the p.d. across the resistor being displayed on the screen of a cathode-ray tube. The effect of the capacitance between the strips is examined analytically, and the capacitance correction to the impedance is found to be about 0.5% at 10 Mc/s, when the length of the bifilar is 50 cm. It is pointed out that the resistor, combined with a valve voltmeter, provides a simple way of checking the calibration of ammeters at frequencies up to about 5 Mc/s. The construction is very robust and so simple that it can readily be made to suit the requirements of each particular application as it arises. It is intended mainly for use as a test-bed tool rather than as an adjunct to a high-precision standard bridge.
