730 THE ELECTRIC BELL. 



quite well, provided that the wires are nowhere placed too near 

 one another. The walls along which the wires run are by no 

 means perfect non-conductors ; a small part of the current will 

 therefore pass through the wall from one wire to another near it. 

 The effect of this is insensible if the wires are 5 or 6 cm from each 

 other, but if they are nearer to one another the action of the current 

 might be seriously disturbed. The wires are generally attached 

 by means of small tacks or wire hooks ; they are driven into the 

 plaster of the brickwork, and the wire is wound round them. It is, 

 however, better to fix with pegs small pieces of wood, 10 or 20 mm 

 thick and 2 or 4 cm wide, at distances of a few metres along the 

 wall, and then to stref-ch the wire by small screws which are fastened 

 into the pieces of wood, so that the wall is nowhere touched by 

 the wire. 



For offices in large buildings, manufactories, etc., the arrangement 

 of the wires is often rather complicated, and in that case several 

 wires are placed side by side, mostly between the wall and the skirt- 

 ing board which runs along the flooring. Whenever wires are to 

 be placed so near each other they must have an insulating cover- 

 ing. Nevertheless, it is usually found that after a time the covering 

 becomes faulty, that the moisture which enters is decomposed by the 

 current passing from one wire to the other through the fault, and 

 that consequently verdigris is deposited and the wire gradually 

 destroyed. The connecting wires should always be visible and easy 

 of access, so that any faulty place may be at once discovered and 

 the fault remedied. 



Where the wires have to pass through the brickwork of walls, it 

 can scarcely be done otherwise than by placing them close together 

 within the opening of the wall. Wires covered with gutta percha, 

 wax, asphaltum, or india-rubber are used in such cases, and the 

 wires placed side by side in a glass tube, which is as long as the 

 opening ; the tube prevents contact between the wires and the 

 brickwork, and protects them against moisture. The covered pieces 

 of wire which project from the tube are soldered to the ends of the 

 uncovered wire, which forms the continuation of the circuit. Simi- 

 larly in all cases where wires have to be joined, this should be done 

 by soldering. 



In fig. 368 various arrangements for the circuit between battery, 

 key, and bell are represented. A is the most simple, such as 

 described previously ; t is the key, 6 the battery, and k the bell. In 

 B there are three keys, t\, t 2 , t 3 , placed at various points of the 

 circuit, as, for example, in three different rooms, from each of which 

 the bell k may be rung. Where wires have to cross each other 



