Fig. 101.—Lightning-Arrester for Telephone Wires.
The lightning-arrester shown in Figure 101 is designed for service on telephone wires. It is an ordinary fuse provided with an arrester in the shape of two carbon blocks about one inch square. The blocks rest on a copper strip, and are held in place by a spring-strip connected to B.
The carbon blocks are separated by a piece of thin sheet-mica, of the same size as the blocks.
The post, B, is connected to one of the telephone-line wires near the point where it enters the building from outdoors. The post, A, is connected to the instrument; C is connected to the ground.
An arrester of this kind should be connected to each one of the telephone wires.
If the line wires should happen to come into contact with a power wire, there is danger of damage to the instruments, but if an arrester is connected in the circuit such an occurrence would be prevented by the blowing out of the fuse. If the lines become charged by lightning, the charge can easily pass over the edge of the mica between the two blocks and into the ground.
CHAPTER VIII ELECTRICAL MEASURING INSTRUMENTS
An instrument designed to measure electromotive force (electrical pressure) is called a voltmeter. An instrument designed to measure volume of current is called an ammeter.