To Measure Internal Resistance of Cell with Queen Acme Set.--First compare its voltage on open circuit with the pressure of the testing set battery. Then, shunt the cell with a known resistance, about 100 ohms, and again measure its terminal voltage. The difference between the two values thus obtained, divided by the value of the shunt resistance, will give the value of the current. To find the internal resistance, multiply the value of the shunt resistance by the ratio between the first and second measured values. [Illustration: Fig. 586.--Evershed portable ohmmeter set. This testing set consists of a direct reading ohmmeter which indicates by direct reading the value of the resistance being tested, also a portable hand dynamo which provides at any required pressure the current necessary to make the test. It is adapted to the needs of supply stations, wiring contractors and dynamo builders. It is also useful in testing the insulation of underground and aerial cables, and is designed so that it can be used by ordinary workmen who are not experienced in handling delicate instruments and who, by its use, are able to obtain accurate results. The dynamo is wound for 100, 200, 500, or 1,000 volts, and is fitted with spring drum inside the case on which is coiled a twin flexible cord provided with a connector adapted for clamping under the ohmmeter terminals.] For instance, assume that the open circuit voltage of the cell being tested as compared with the voltage of the testing set battery is .212 of the latter, and that when it is shunted with a resistance of 1,000 ohms, its terminal voltage is .179. Then, the total resistance is to the 1,000 ohms shunt resistance as .212 is to .179 or (.212/.179) × 1,000 = 1,184, from which deducting the 1,000 ohms shunt resistance, gives 184 ohms as the internal resistance of the cell. [Illustration: Fig. 587.--Leeds and Northrup fault finder. A lineman's instrument for the location of faults, crosses, grounds, and opens in telephone and telegraph circuits, and for the measurement of conductor and insulation resistance.]