502 INSTALLATION OF ARC LAMPS [Cn. XIII 



696. Selection of material for installing the arc lamp. After 

 determining the maximum amount of current needed for the arc 

 lamp, then the wire of proper size and quality and insulation to 

 conform with the National Electrical Code should be obtained. 

 The simplest way to do this is to go to some reliable dealer in elec- 

 trical supplies and get the standard material.' 



Standard switches, etc., are all marked plainly so that there is no 

 difficulty in selecting the correct sizes. In America, wire is more 

 often designated by some standard wire gauge, e. g., that of Brown 

 & Sharp, than by the actual diameter in millimeters or inches. In 

 the above table the sizes in millimeters and inches corresponding 

 with the B & S gauge numbers are given, also the area measured in 

 circular mils. 



One must not forget that everything that is used wears out, and 

 when any piece of apparatus or the wire becomes deteriorated by 

 use it should be replaced. 



WIRING FOR THE ARC LAMP, THE RHEOSTAT OR OTHER BALANCING 

 DEVICE, AND THE LAMP SWITCH 



697. Connection with the electric supply. It is assumed that 

 the electric supply has been properly installed by an electric com- 

 pany, or from a private dynamo, to within a short distance of the 

 arc lamp. This supply will be in a proper outlet box, with fuses 

 and switches in accordance with the National Electrical Code. In 

 case the outlet box is on the wall close to the arc lamp, the simplest 

 and most convenient connection between the lamp switch and the 

 supply in the outlet box is by means of a separable attachment of 

 the proper capacity for the maximum current. (See the table of 

 flexible cables, 695.) If the current is direct, then it is a conve- 

 nience to have this attachment irreversible, or polarized so that 



For No. 18 wire, as before, the circular mils are 1624 and the relative carrying 



capacity is assumed to be = ^' 2I , whence X = .116 amperes. If there 



3 X 



are 40 wires in each cord then each cord should carry .1 16 x 40 = 4.64 amperes, 

 or in round numbers 5 amperes. If the double cord were used for each conduc- 

 tor to the lamp, then in like manner twice as much could be carried, as there 

 are 80 wires: .116 x 80 = 9.28 amperes or 10 amperes in round numbers. 



