This is shown in Fig. 64; its use is to change the direction of the currents through - the primary and secondary circuits. It consists of an ivory or ebonite cylinder, 1 in. long and 1 in. diameter. Metallic axes project from each end in separate pieces, 2 brass plates J in. wide are fixed to opposite sides of the cylinder, one connected with each axis. The cylinder is supported horizontally on 2 brass blocks or pillars fixed to the base - board; and 2 brass springs rise up from the board and press on the brass plates on the face of the cylinder. Of the 4 binding - screws on the board, 2 are connected with the 2 springs by wires passing underneath or over the base - board,and the other 2 with the blocks carrying the axes of the cylinder. One axis projects through the block in which it rests, and on it is fitted an ivory or ebonite plate, to enable the cylinder to turn round. Two of the binding - screws on the board are connected with the battery, and the other 2 with the apparatus to be operated with.

Fig. 64.

Commutator 30067

The current passes from the anode of the battery, through one binding - screw of the commutator, under or over the board, to one of the springs, up this to the plate on the cylinder it is in contact with, to the first axis, through the block in which the axis rests, and out by the binding - screw connected with that block to the apparatus, returning by the other spring plate and axis to the battery. When the cylinder is turned half round, without any other change in the arrangement, the current will pass in the opposite direction, still from the anode of the battery to the first spring, but thence to the second axis.

If the commutator be mounted on the pedestal, separate stand and binding - screws will not be required. It is capable of being used also as a current suspender: if the cylinder be turned only 1/4 revolution, the springs rest upon the interspace between the 2 brass plates, and contact is broken. This should be done while the arrangements for the secondary current are being made, to avoid receiving a shock.

Fig. 65 shows Ruhmkorff's commutator. The battery poles are connected through the ends of the axis of a small ebonite or ivory cylinder to 2 brass cheeks, V V, which can be turned so as to place them either way in contact with 2 vertical springs B C, which are joined to the ends of the primary coil.

Fig. 66.

Commutator 30068

Connections

As to the way of making the connections beneath the base - board, it will be assumed that the commutator is not fixed on it. Then on turning the pedestal of the coil upside down, the under side will present the appearance shown in Fig. 66 : a b are the ends of the binding screws to which the battery is attached; c, pillar of the contact - breaker that carries the platinum screw; d, end of block that carries the spring forming the" other part of the contact - breaker; e f, beginning and end of primary coil. A loose board g fits into the pedestal, as shown by the dotted lines, and' is fixed about half - way up from the bottom. This is required to lay the condenser on, and to afford convenient means for attaching it. On it, at h i, 2 flat brass plates, about 1 in. square, are fixed, having a screwed pin in the centre of each. These are for connecting the condenser with the contact - breaker. The various screws project through the top of the pedestal fully 3/8 in. to enable nuts to be screwed over them. Strips of sheet copper, about 1/2 in. wide, are prepared, 2 having a hole at one end and binding - screw at the other, and the third having a hole at each end.

This latter is long enough to connect b and c, by dropping it over their projecting screws, and screwing a nut down tightly on the copper, securing the strip, and making the contact. The strips with binding - screws are used to connect a e and d /, the binding - screws securing the wires e f, and the holes in the copper strips enabling them to pass over the projecting screws d a, where they are fixed by nuts. When the anode of the battery is attached to the binding - screw at a, and the cathode to that at b, the current will flow through the coil. The circuit is complete in the direction of the dotted line a e, through the primary coil, out at f, then from f to d, through the contact - breaker to c, and from c to b.

Fig. 66.

Connections 30069

The condenser is laid on the loose board g, with the projecting tinfoils resting on the brass plates h i, a hole being punched through each set of tinfoils to admit the screw - pin; a brass plate is then laid on the top of the tinfoils, and by means of a nut on the screw - pin they are pressed closely together. When the condenser has been fitted in place, the nuts are loosened, the top brass plate is taken off, and 2 copper strips with holes at each end are fitted over the pins. These copper strips must be long enough to connect the pin h of the condenser with c of the contact - breaker and the pin i with d; this done, the nut is screwed up again. If the condenser is not thick enough to fill the space between the loose board and the bottom, the latter is padded, so that it will press on the former, and prevent it from shifting.