Assuming that the resistance of the instrument is 8,000 ohms and that the current deflects the needle through 10 divisions of the scale, then for a deflection of one division the resistance is 8,000 × 10 = 80,000 ohms. Accordingly, if, when the switch is moved to the right, connecting the voltmeter across the unknown resistance, the needle be moved through 6 divisions of the scale, the combined resistance of the voltmeter and unknown resistance is 80,000 ÷ 6 = 13,333-1/3 ohms, and subtracting the resistance of the voltmeter, the value of the unknown resistance is 13,333-1/3 - 8,000 = 5,333-1/3 ohms. Ques. For what kinds of test is the voltmeter method best adapted? Ans. For measuring high resistances, as the insulation of wires, etc. Ques. What may be said with respect to the current used? Ans. Its voltage should be as high as possible within the limits of the voltmeter scale. [Illustration: Fig. 563.--Standard resistance box: 100,000 ohms, in four units of 10,000, 20,000, 30,000, and 40,000 ohms. An "infinity" plug separates each coil from the ones adjacent. Segments are elevated from the hard rubber top by special washers in order to increase insulation. Binding posts are so arranged as not to be in the way when plugs are used.] [Illustration: Fig. 562.--Standard high resistance box: 100,000 ohms. It is mounted in a brass box with a hard rubber top. Connections should be made to terminals marked 3 and 4. When the flexible cord is on plug 1, the box is short circuited, but when on plug 2, the resistance of 100,000 ohms is in series. The box is especially suited to rapid cable testing.] Ques. In testing cable insulation what is desirable with respect to voltmeter and current? Ans. A low reading voltmeter should be used in connection with a large battery. [Illustration: Fig. 564.--Diagram showing principle of Wheatstone's bridge. A, B, C, and D, are the four members which constitute the bridge. The current from the battery divides at P, part traversing DC, and part traversing BA. The galvanometer connected to M and N will indicate when the currents are equal in the two branches by giving _no_ deflection. This is then a _zero_ or _nil method_ of testing. The resistances and keys required in testing are shown in fig. 565. In the actual instrument, the members A, B, C, and D are known by the names given in the figure.] Wheatstone Bridge Method.--For accurate measurements of resistance this method is almost universally used. The so-called "Wheatstone" bridge was invented by Christie, and improperly credited to Wheatstone, who simply applied Christie's invention to the measurement of resistances. [Illustration: Fig. 565.--Diagram showing arms of Wheatstone bridge, resistances and method of connecting galvanometer, battery and unknown resistance.]