Arc lighting.

is arranged so as to feed into a single circuit, and also because circuit breakers and voltmeters are not required. A simple arc light board for four generators is shown in Fig. 50. It consists of a marble slab which carries eight plug receptacles at the upper edge, and eight more at the bottom, and an ammeter in the central space. Eight wires

running from the terminals of four machines are connected with the eight lower receptacles, and eight line wires are connected with the eight upper receptacles. By means of the flexible cables shown, any one of the lower receptacles can be connected with any one of the upper ones, as is clearly indicated in the illustration. A

Arc lighting.

voltmeter is not required, owing to the fact that the e. m. f. of the generator is continually rising and falling in accordance with the demands of the circuit.

If the num

ber of lamps in service is increased, the e. m. f. must correspondingly increase, and if the number of lamps is re

Arc lighting.

duced, the voltage must also reduce.

This variation in the e. m. f. is effected by the regulator of the machine. An ammeter is required, however, because the current must remain constant to keep the lights from varying in brilliancy. In the board shown in Fig. 50 there is only one ammeter; therefore, this can only be used from time to time to test the several circuits and ascertain whether the current strength is what it should be or not. In more complete and expensive boards, one ammeter is provided for each circuit, and this is left in service all the time so that the strength of the current may be observed without going to the trouble of cutting the instrument into the circuit.

The switchboard connections of an arc light system are clearly shown in Fig. 51, which is so simple a diagram that an extended explanation is not necessary. It will be noticed, however, that the number of receptacles or sockets is greater than in Fig. 50, there being five rows at the top and five at the bottom. The advantage of this arrangement is that by means of two short cables a machine whose terminals run to one end of the board can be connected with a circuit whose terminals are connected with the other end, and thus the confusion arising from the use of long cables is avoided. If there are only a few machines, this construction is of no particular advantage, but in cases where a large number of circuits are to be provided for it is.

In Fig. 51 the first machine is arranged to feed into one circuit only, the second machine supplies two circuits that are connected in series, and the third machine supplies three circuits connected in series. This way of connecting the circuits is not often used at the present

Arc lighting.

time, but was quite common in former days when small size generators, ranging from sixteen to twenty-five lights, were to be found in most arc-lighting stations. With such machines there is no special objection to connecting a number in series as their combined e. m. f. is not more than that of one large generator. The connection of the circuits in series in the diagram serves to illustrate clearly the advantage of providing a number of sockets connected with each other in the manner indicated by the horizontal lines. As will be noticed, the second and third circuits are connected in series with each other, and Iwith machine No. 2 without the aid of the horizontally connected sockets; but the last three circuits and the third machine are connected in series through the lower middle row of sockets by the use of two short cables, one running from the lower No. 6 terminal, and the other from the left side of the generator. The second machine could also be connected in this manner, as lower terminal No. 3 could be connected with the lower row of sockets, and the generator could be connected with the top row. In this particular case such an arrangement would be of no special advantage, as the cable from the lower machine terminal to lower 3 is not long and does not cross any of the other cables. In the case of the third machine, however, if a cable were run directly from lower 6 to the machine terminal, two cables would be crossed. As can be seen from this explanation, if the board were made of sufficient size to accommodate twenty or more generators, with an equal number, or more of circuits, the complication arising from direct cable connection from the machine to the circuit terminals would be very great. All this complication could be avoided if