4Q6 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 capac- 

 R ^/ ity of conducting wires is in genr 



eral proportional to their size, 

 FIG. 265. SHORT CIRCUIT. hen(;e for , arge mmnts it is 



G ,c G Snd a uSo,ex d ,Sg across the ^cessary to have larger wires 



circuit making the path back to the than for small currents (see the 



dynamo (G) shorter than the course , i i -i i e < \ 



through the arc lamp (A) and the rheo- table below 694). 



If a wire is put across the points s 688. Short circuit. By a 



and c the electricity will take that path s h O rt circuit is meant the short- 



mstead of the longer path through the ,. , . , , 



arc. enmg of the distance which the 



