512 WIRING FOR ALTERNATING CURRENT [Cn. XIII 



moist, many small bubbles will appear around the negative pole. 

 But the greenish color given at the positive pole is the most certain. 

 Turn off the current and mark the positive wire red. 



With the other chemical tests (A, B, C) the indications are in no 

 way dependent on the metal forming the wire, but with the potato 

 test the poles entering the potato must be copper or contain 

 copper. 



704. Wiring the three-wire automatic lamp of the Bausch & 

 Lomb Optical Company. This lamp is regulated wholly by elec- 

 tricity, there being no clock-work. In wiring the lamp one pro- 

 ceeds exactly as described above ( 693-700), except that a wire 

 is carried from the positive side of the switch to the middle binding 

 post of the lamp directly. Another wire from the same point is 

 carried down to the resistor or rheostat, and from the rheostat a 

 wire to the positive or upper binding post of the lamp. From the 

 negative pole of the switch a wire is. carried directly to the lower 

 or negative binding post of the lamp. This wiring gives the full 

 voltage of the line for the electric mechanism governing the lamp 

 (see fig. 145). 



WIRING FOR ALTERNATING CURRENT 



705. This is precisely as for direct current, and one does not 

 have any trouble about the polarity. It makes no difference 

 which supply wire is connected with the upper carbon and which 

 with the lower. 



706. Insertion of the rheostat, inductor or other balancing 

 device. It makes no difference in which of the lead wires the 

 rheostat, etc., are inserted. Just as with direct current, however, 

 the balancing device must be inserted along one wire (fig. 281), 

 otherwise the current would not traverse the entire circuit. 



707. Position of the rheostat, etc. The balancing effect of 

 the rheostat is the same no matter where it is installed in the special 

 circuit for the arc lamp. For convenience it is frequently put on 

 or near the projection table. This is especially necessary if the 

 rheostat is adjustable. With a fixed rheostat it is sometimes safer 



