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496
WIRING FOR AN ELECTRIC CURRENT
[Cn. XIII
WIRING FOR AN ELECTRIC CIRCUIT FROM THE DYNAMO BACK TO
THE DYNAMO
§ 686. For the purposes of projection by the aid of an arc lamp, the electric current required, whether it be direct current or alternating current, is practically always furnished by a dynamo. To make the electricity available there is a conductor of some kind, usually a copper wire extending from one pole of the dynamo to the arc lamp or lamps, and from them back to the other pole of the dynamo. Such a loop of wire from pole to pole of the dynamo forms an electric circuit, regardless of the length of the wire. With direct current, any part of the wire nearer the positive pole of the dynamo is positive to any part of the wire nearer the negative pole of the dynamo, hence the wire extending out from the positive pole of the dynamo is often designated the positive wire, and the wire received into the negative pole of the dynamo is called the negative wire. It will be seen from fig. 275, 280 that the circuit is a loop of wire with the two ends connected with the two poles of the dynamo.
With alternating current, as stated above, there is no constant polarity, hence it is not correct to speak of negative and positive wires or positive and negative poles of the alternating current dynamo.
§ 687. Amperage for different purposes. — As the quantity of
electricity needed for different purposes varies, the capacity of the generator or dynamo must vary. Also the carrying capacity of conducting wires is in general proportional to their size, hence for large currents it is necessary to have larger wires
circuit making the path back to the than for small currents (see the dynamo (G) shorter than the course , 11 •, -\ g , \ through the arc lamp (A) and the rheo table below § 694)
If a wire is put across the points <> § 688. Short circuit.— By a
and c the electricity will take that path short circuit js meant the short
mstead of the longer path through the
arc. emng of the distance which the