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BELL SYSTEM TECHNICAL JOURNAL 



the relay the condenser is charged suddenly to the applied voltage, the 

 charging current passing through the lamp filament. This current, at 

 the outset, is several times the normal current for the lamp and brings 

 it to full brilliancy quickly. The resistance shunting the condenser 

 has such a value that normal current flows through the lamp filament 

 in the steady state, so, once lighted, the lamp remains at normal 

 brilliancy as long as the circuit is closed. The lamp may be lighted in 

 even less time if a small current is left flowing through the filament 

 continuously in order to keep it hot but not hot enough to be luminous. 



DRIVE 



DRIVING 



Fig. 13 — Sectional scale drawing of magnetic clutch. 



A resistance of suitable value connected as indicated by dotted lines 

 in Fig. 11 will accomplish this result. 



The magnetic clutch, while especially designed to operate quickly, 

 accelerates the sprocket and film without shock in order to avoid 

 danger of tearing the film and to reduce wear and tear on the mechan- 

 ism. The clutch is shown mounted on the oscillograph in Fig. 12. 

 It is also shown diagrammatically in Fig. 13 to illustrate its construc- 

 tion and operation. The annular coil in the driving member is con- 

 nected, through slip-rings and contacts on the high-speed relay, to a 

 battery. When current flows in this coil a steel diaphragm on the 

 driven member is drawn against the annular electromagnet, traction 

 being obtained at the outer edge of the diaphragm. Due to the small 

 clearance between the diaphragm and the electromagnet the diaphragm 

 is drawn into contact very quickly, and due to the small moment of 



