THE MAGNETO TELEPHONE 135 



this a line fuse must be introduced 1 in the line between the protector and the outside 

 wires. This fuse contains a short length of easily fusible metal which melts when- 

 ever a current of greater strength than the fuse is designed to carry comes over the 

 wire. The circuit is thus opened and damage by sparking is prevented, as the air 

 gap through the fuse is entirely too large for any ordinary high-tension current to 

 jump. Fig. 84 shows a protector designed to guard against both lightning, and power 

 and light currents which has bean adopted by the Forestry Branch for use wherever 

 both kinds of dangerous currents are encountered. 



This means that on many reserves all instruments must have the combined light- 

 ning and high-power protector, because no matter on what part of the circuit a power 

 line is encountered it is necessary that all instruments oil that circuit be fully protected. 



Fig. 85 Baby knife-switches. (A) Single pole, single throw (S.P.S.T.). (B) Double pole, 



double throw (D.P.D.T.) 



(J) Switches. For convenience in separating a branch line from a main line, 

 for cutting off stations not in use, for dividing extra long lines into shorter sections, 

 and for other similar purposes, it is frequently necessary to install switches in telephone 

 circuits. As the currents carried are all comparatively weak, no very elaborate 

 switching devices are needed, and a simple, inexpensive form of knife-switch is found 

 to serve all purposes adequately. Those mounted on <a porcelain base as shown in 

 Fig. 85 will be found most satisfactory. They are fastened in place with round-head, 

 blued screws of suitable gauge and length. 



Switches of this kind are classified by the number of poles either as single, double, 

 triple, etc. They are also further divided into two classes, depending on whether they 

 make one contact or two. Those making but one contact are called " single-throw " 

 (S.T.); those 'making two, "double-throw" (D.T.). For most purposes a single-pole, 

 single-throw switch ('S.P.S.T.) is sufficient (see Fig. 85). Where more complex con- 

 nections are involved various multiple-pole switches of either the S.T. or D.T. type 

 may be required. It is always to be remembered in placing switches that the more 

 connections that can be made or broken by the movement of one switch, the less 

 likelihood there is of confusion in operating the switchboard. Switchboards should 

 always be designed, therefore, to simplify as much as possible the movements required 

 by using multiple-pole switches wherever feasible. 



(e} Repeating coils. These are simply a special form of induction coil generally 

 employed for the purpose of preventing the unbalancing of metallic lines when joining 

 grounded lines to them. While it is possible to neutralize inductive disturbance on 

 metallic lines by proper transposition, this effect is lost if a grounded line is joined 

 directly to a metallic line without interposing a repeating coil. By inserting a repeating- 



