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.
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.
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.