Large switches.

The switch Fig. 101 is called a single-throw switch, because the blade can only be thrown in one direction to close the circuit. The design shown in Fig. 102 is called a double throw switch from the fact that the blade will close the circuit if thrown in either direction. If the switch has but one blade it can only open or close the

Figure 1. Switch for heavy current.

circuit in one wire, and is, therefore, called a single-pole switch; but if S consists of two blades, as shown in Fig 103, which is a plan of Fig. 102, then if the two blades are insulated from each other, each can open or close the

Quick break switches.

circuit through one wire. Switches of this latter type are called double-pole, and if S consists of three insulated blades the switch is three-pole.

In Fig. 102, as well as in Fig. 101, the current passes through A, therefore there must be a perfect fit between A and S. In Fig. 104 the current does not enter through A, hence a loose fit will answer as well as a tight one. In this last figure the current passes through the blade S

from N to N; therefore at these points a good fit is required, and N must exert a considerable pressure against S. It can be readily seen that if the blade S is raised in Fig. 104 it will produce two breaks in the circuit. On

How they work.

this account switches of this construction are called double-break switches.

If the two sides of S in Fig 103, are not insulated from each other, the switch cannot be used as a doublepole. If the voltage of the current that passes through the two sides of a double-pole switch is low, the insulation need not be very heavy, but for an electro-motive

force greater than 110 volts the most substantial insulation must be provided. When a two-pole switch is used to connect a generator with the switchboard, the outgoing current passes through one of the blades S and the returning current passes through the other, and since it is much easier for the current to jump across from one side of the switch to the other than to make the journey through a

Carbon break switches.

weak point in the insulation. To properly insulate the sides of a multiple pole switches several types of construction are resorted to, one of which is shown in Fig.

103. This consists in providing a block, which is made of hard rubber, fiber or hard wood, the former being used for the higher grade of switches and for higher volt

Sparks with heavy current.

Other

ages. The sides S S are bent around at right angles at the ends and are secured to H by means of screws. constructions are shown in Figs. 105 to IIO.

Figs. 108 and 109 show constructions for three-pole blades, and Fig. 110 is the type employed for switches of large capacity. When a strong current has to be transmitted through a switch, it is not sufficient to make the several parts of sufficient cross-section to carry the current without becoming overheated, but it is also necessary to provide a sufficient amount of contact surface, for if this is not provided the heat developed at the points of contact may be enough to absorb a considerable amount of energy and in extreme cases, may even result in burning out the switch, with more or less serious results. By providing three blades side by side for each pole, as in Fig. 110, the surface of contact is increased three-fold. The actual construction of large switches of this kind can be better understood from Fig. 111, which is a switch made by La Roach & Co., of 3.000 amperes capacity.

When a switch is opened, through which a strong current is passing, a large spark is formed as the switch blades leave the contact, and the effect of this spark is to burn away the metal of both parts. This burning not only results in gradually consuming the working parts of the switch, but by roughening up the surface destroys the perfect contact. The trouble can be remedied by smoothing off the burned portions with a file, but this procedure, in the hands of a man who is not a good mechanic, soon spoils the switch by destroying the fit between the contact surfaces. The extent to which the parts are burned by the spark is dependent not only upon the strength of the current, but also upon the duration of the spark; therefore, if the time during which the