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5U SWITCHES, FUSES, CIRCUIT BREAKERS [Cn. XIII
If it is difficult to get double cord of the right size, then each of the wires to the lamp can be composed of the double cable. This is easily done by removing the insulation at each end of the double cord and twisting both the wires together. (See the tables § 694, 695, for the carrying capacity of flexible cord and cables).
§ 710. Wiring the arc lamp with a three-wire supply. — Only two wires go to the arc lamp, then if one must connect the arc lamp for projection to a three-wire supply system it is necessary to remember that the middle (neutral) wire and either outer wire will give no volts the same as the two-wire no volt circuit.
If connection is made with the two outer wires then 220 volts will be used in the arc lamp. In this case a rheostat for a 220 volt circuit must be employed, or two no volt rheostats in series (fig. 287).
Naturally one would connect 'with the middle or neutral and an outside wire and employ the usual no volt rheostat but for the fact that such an arrangement would badly unbalance the work of the line, and might cause trouble if the electric circuit was running nearly on full load. It is therefore safer to connect with the outside wires and use the requisite amount of ballast.
SWITCHES, CIRCUIT BREAKERS AND FUSES; THEIR CHARACTER,
INSTALLATION AND USE
§ 711. A switch is a device by means of which a gap (fig. 275 and 276) can be made in an electric circuit thus stopping the flow
of current.
A switch should be so con
structed that when it is opened
\ it makes a gap in all the wires B of the circuit. For example, in J a two -wire circuit, the switch should make a gap in both wires,
FIG. 275. CIRCUIT WITH A BREAK and in a three-wire circuit, a gap
OR GAP. • ir ., . TJ 1
Unless the metallic circuit, from the m a" three ™* . If SUch a
dynamo, G, back to the dynamo, is switch IS used the line beyond
complete, no current will flow. A gap the switch ig "dead, "and no CUTin the circuit (B) prevents the flow of
current. rent can be drawn from it.