The two outside pieces are each fitted with two binding-posts, and the center triangular-shaped piece is fitted with one post. A hole about one-eighth of an inch in diameter is bored between each of the metal pieces. Make a tapered metal pin which can be placed tightly in the holes, and will make contact between the metal pieces. The two outside line wires of the telegraph circuit are connected to the outside metal pieces _C_ and _B_. _A_ is connected to the earth or ground. In case of a lightning storm, if the wires become charged, the small space between the metal plates will permit the charge to jump across and pass harmlessly into the ground. If complete protection is desired, it is merely necessary to insert the plug in one of the holes, and thus "ground" either wire or short-circuit both of them. [Illustration: 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_. There are many forms of reliable meters for measuring current and voltage, but all are more or less expensive and out of the reach of an ordinary boy. Some meters are more carefully made than a watch, and are provided with fine hair-springs and jeweled bearings, but all depend upon the same principle for their action, namely, the mutual effects produced between a magnetic needle and a coil of insulated wire carrying a current of electricity. The little meters described in this chapter are simple and inexpensive but quite sensitive. Unlike a meter making use of a hair-spring, they will stand considerable rough handling, but of course should not be subjected to such treatment unnecessarily. Two types of meters are described. Both operate on exactly the same principle, but one is more elaborate than the other.