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Sparks with heavy current.

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 1IO.

Other

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

Strain on insulation.

spark holds out can be reduced, its destructive effects can be reduced. To reduce the duration of the spark switches are made so as to move quickly after they break contact, and the way in which the result is accomplished can be understood from Figs. 112, 113 and 114. Switches of this kind are called quick-break or snap switches.

Fig. 115 shows a three-pole switch of the type shown in Fig. 114, the handle being raised to the point where the side bars are about to leave the contacts N. Quick action switches succeed in reducing the destructive effect of the spark upon the switch, but are not very desirable so far as the generator and motors in the circuit are concerned, owing to the fact that if the duration of the spark is reduced the strain upon the insulation of the generators and motors is increased. When a circuit is opened the current tends to keep on flowing, and as the regular channel is disconnected, it tries to complete a circuit through some other path. If there are weak points in the insulation they will be pierced by the current in its effort to close the circuit. The action of an electric current in this respect can be compared to that of a current of water brought to a sudden stop. Now, in the case of the water current, if the flow is checked instantly the force exerted by the current becomes very great, but if the flow is stopped gradually the reverse is the case. With electric currents the result is exactly the same, if the current passes through wire coils wound upon a mechanism. If the current dies out slowly the kick, as it is commonly called, is slight; but an instantaneous stop may produce an enormous reaction, even if the current is small.

From this it will be seen that quick-action switches are objectionable because they increase the strain brought to bear upon the insulation when the current is inter

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Different kinds of contacts.

rupted. To obviate the objection of the quick break and at the same time protect the switches from the destructive action of prolonged sparks, the type of switch shown in Figs. 116, 117, 118 is used. In this switch two sets of contacts are provided, one being made of copper and the other of carbon blocks. The first set of contacts is for the purpose of conducting the current with the least resistance possible, and the second set is to receive the spark at the instant when the current breaks.

Switches of the type shown in Figs. 116, 117, 118 are made in many designs, but they are not used as extensively as formerly, except in connection with circuit breakers. For although they prevent the destructive action of the sparking upon the switch blades and contacts, they are rather bulky and more liable to get out of order than the plain type.

The stationary contacts N, and the blades S of switches should be made so as to fit well, and also so that when the blade is lowered it will not be caught upon the ends of the jaws. Some manufacturers provide against the latter difficulty by bending the ends of the jaws outwardly, as in Fig. 119, while others leave them straight as in Fig. 120, and bevel corners of the blade. The most important point about the jaws and of switches is to make them so that they will fit well. If the jaws are made long, they may apparently fit well when in reality they do not. To obtain a good fit, the part b to which the jaws are fasened should be of the same width as the blade S. The jaws should be made of sheet copper rolled hard so as to be springy. They should be bent at the upper edge of b, but only enough to slightly reduce the width of the opening at the top ends.

With such construction, if S and b are of the same

Examples of switches.

width, the jaws will stand parallel when the blade is in place, and there will be a bearing from edge to edge of S. If S is wider than b, as shown on Fig. 121, the effect will be as shown in Fig. 122 when the blade is in place; and if S is narrower, as shown in Fig. 123, the fit when

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Fig. 127.

Fig. 128.

Two Crouse-Hinds switches.

the blade is in place will be as illustrated in Fig. 124. The thickness of the jaws should be sufficient to make them decidedly stiff, so that they may have to inward a very small amount to produce the necessary clamping pressure. Some switch jaws are made of the form illus

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