CH. XI] MOVING PICTURE APPARATUS 399 



559. Wiring. When the supply is decided upon, the wiring 

 is next installed. This must be heavy enough to carry the greatest 

 current which is to be used continuously in the lamp. It does not 

 need to be designed for the rather high current which flows when 

 the carbons are brought into contact, as any wiring can withstand 

 a heavy overload for a few seconds without injury. 



The wire which enters the lamp-house should be flexible cable, 

 asbestos covered, and of a carrying capacity at least double the 

 amount required for use at the arc ( 694-695) . This is on account 

 of the high temperature within the lamp-house and consequent 

 rapid deterioration of a small wire. 



560. Fuses. Fuses should be used in every case and not 

 circuit breakers. This is because a fuse will not "blow" instantly 

 when current is drawn greater than its normal capacity (as when 

 the arc is started) but if this overload is continued, it will melt and 

 open the circuit. The circuit breaker, on the other hand, will open 

 the circuit instantly at the same amperage whether the current is 

 momentary or long continued. 



561. Fire underwriters and special regulations. The wiring 

 and installation must conform to the fire underwriters regulations 

 and any special requirements of the city in which the theater is 

 located. The wiring for moving picture machines is neither 

 heavier nor more difficult to install than that required for other 

 forms of projection, notably opaque projection, provision for 25 

 amperes direct current or 50 amperes alternating current usually 

 being sufficient for small theaters. 



For currents required in different cases; for the size of wire 

 required for these currents and for fire underwriters regulations see 

 Chapter XIII, 691. 



562. Rheostat or other ballast. As with all forms of arc 

 lamp, the moving picture lamp requires some form of ballast or 

 regulating device to control the current. 



The simplest and cheapest device is of course the resistor or 

 rheostat. When the electric supply is no volt direct current, a 

 rheostat is generally used. A rotary motor-generator set or 



