[Illustration: Fig. 584.--Kelvin bridge. This includes a low resistance standard of .1 ohm variable by steps of .00001 ohm, a set of ratio coils, and a holder for rods or wires to be measured, with a scale to measure their length. It is also provided with heavy flexibles to be used in measuring the resistances of irregularly shaped pieces. The connections are clearly shown in the diagram. The range of measurements of this bridge is: 1 ohm to .1 ohm by steps of .001 ohm readily estimated to .0001; .1 to .01 ohm by steps of .0001 ohm readily estimated to .00001; .01 ohm to .001 ohm by steps of .00001 ohm, readily estimated to .000001; .001 ohm down by steps of .00001 ohm, readily estimated to .000001 ohm.] Change the rheostat resistance accordingly until the balance attained is such that a very slight variation of the rheostat resistance one way or the other will reverse the galvanometer deflection. To find the pressure on the circuit, add 100 to rheostat resistance and point off two places. Multiply this value by the voltage and the product will be the desired voltage. If the voltage of the standard cell be exactly one volt, the total employed resistance represents the voltage on the circuit. [Illustration: Fig. 585.--Queen slide wire bridge. It consists of a portable slide wire, Wheatstone bridge arranged to read directly in ohms in addition to its use for locating crosses and grounds. It is complete with battery, galvanometer and telephone receiver. The bridge is balanced by moving the hand stylus until the galvanometer shows no deflection or until there is no sound in the telephone receiver. In order to provide a wide range of measurement and maximum accuracy, ratio coils or multipliers having values of 1, 10, 100, 1,000 and 10,000 are provided. The scale of the instrument is arranged in two parts, one of which indicates ohms and the other is divided into uniform divisions for use when locating crosses and grounds by the Murray and Varley loop methods. A small induction coil is included so as to furnish an alternating current when using the telephone receiver.] For instance, in making a measurement on a 110 volt circuit, assume that the employing of 7,840 ohms rheostat resistance produces balance, and that increasing or decreasing this resistance by two ohms, reverses the galvanometer deflection. This indication that the setting 7,840 is uncertain, about 1/40 of 1 per cent. Since the rheostat coils are adjusted to an accuracy of only 1/5 of 1 per cent., that will be about the accuracy of the measurement. If the pressure of the standard cell be 1.018 volts, then 7,840 + 100 = 7,940. Pointing off two places, gives 79.40, which multiplied by 1.018 gives 80.82 for the voltage on the circuit.