Oil break switches.

trated in Fig. 125, this construction being used under the erroneous impression that if there is sufficient elasticity, a bearing can be obtained over the entire width of the switch blade regardless whether the fit is good or bad. This notion is not correct; the best results are obtained with short jaws, but the perfect fit is a matter of accurate workmanship.

As the resistance of the contact surface of the switch jaws is much greater than that of the body of the conductors, an extra amount of heat is developed at these points, and for this reason it is desirable to have as much metal near the contacts as possible. One way of increasing the bulk of metal is shown in Fig. 126. It is very commonly used in switches of the side-throw type, in which case the thick side of the jaw rests upon the surface to which the switch is attached, and the spring is placed on top. Some of the commercial forms of knife switches are shown in Figs. 126 to 129.

For the high voltages used in alternating current circuits, it is necessary to devise means whereby the sparking produced when the switch is opened may be reduced to small magnitude, if this construction is not employed, then the contacts must be of such form that they will withstand heavy sparking without being seriously injured. To prevent the sparks from attaining a destructive magnitude, several expedients are resorted to, one of which is to place the contact point in a tank of oil. When switch is so arranged, the spark at the instant of opening the circuit is very small because the oil is a very high insulator, and being liquid, it immediately falls into the space left by the switch contacts, and thus, by interposing an insulator or very high resistance in the break between the contact points, the flow of the current is at

How they work.

once stopped. Even with an e. m. f. of two or three thousand volts, the spark produced when the contacts are separated under oil are very short, probably not more than a sixteenth of an inch. Such small sparks are necessarily of short duration, so that the burning effect upon the switch terminals is very slight.

Some switches are arranged so that the contacts are separated under oil. This construction is illustrated in the vertical elevation Fig. 130. The box which holds the oil, and within which the switch is placed, is made of iron. and varies in dimensions according to the capacity

Power operated switches.

of the switch. For a switch of about 25 amperes, and an e.m.f. of 2,500 volts, the size is about 6 x 4 x 7 inches. The current enters the box just above the level of the oil and passes down through side posts to bushings held on the upper side of a slate slab. The switch carries plugs. which slide through these bushings and into tubes located directly under them and secured to the under side of the slab. The number of the plugs varies with the e.m.f. of the current, being generally sufficient to make voltage at each break not over 800. The tubes and the plugs are properly connected with each other so as to form a continuous circuit when the switch is in the closed position. The switch is moved by a hand lever controling central rod.

There is also another plan in use for reducing the size of the spark. In this design the switch contacts separate within a narrow passage, and on account of the restricted size of it the arc formed cannot be of very large dimensions. The switch is operated by pulling out the handle and the frame that holds the switch contacts is guided by the posts as is clearly shown in the figures.

A very similar switch is shown in switchboards, Figs. 75 and 76, shown in the latter figure in the form of a cross above the rheostats.

Switches intended for carrying very strong currents are in some instances arranged so as to be operated by compressed air. A switch of this type is used at Niagara Falls. The operation is the same, except that a compressed air cylinder controls the movement. A very high resistance is interposed between the first and second set of contacts, so that when the first break, the current is immediately cut down by the increased resistance in the

1

Power operated switches.

circuit, and when the final break occurs, the strength of the current is much reduced. The switch shown in the illustration is of the four-pole type, and at each pole there is one knife break and eight tubular breaks. The switch is used in connection with two phase currents, and there are two poles for each circuit, hence at the primary break the circuit is interrupted at two points, and at the final break it is interrupted at sixteen points, so that the e.m.f. per break is one-sixteenth of the total line e.m.f.

Circuit Breakers.

A circuit breaker is a switch so constructed that it will open the circuit automatically when the current reaches the strength at which the device is set to operate. A circuit breaker in its most elementary form is shown in Fig. 131. The coil of the magnet M is connected in series in the circuit that is to be protected. The core C of this magnet exerts an attractive force upon the end A of the lever L, and when the current strength rises to a certain point the attraction becomes sufficient to overcome the weight of the end S. When this occurs, A is drawn against the end of C and S separates from N, thus opening the circuit. If a sliding weight is placed upon the end S of the lever, of if in addition to the weight of the tension of a spring, pulling downward, has to be overcome, it is evident that by moving the weight further from the fulcrum B, or by increasing the tension of the spring, the resisting force can be increased, and thus the device can be adjusted so that O will be drawn against C with such strength of current as may be desired. To make the apparatus a circuit breaker it is necessary to provide the catch t, for without this it would be simply a current interrupter. As can be readily understood, if there were no catch the lever would drop away from C soon after being attracted, for the separation of S from N would interrupt the current and therefore the strength