PIERCE. THE COOPER HEWITT MERCURY INTERRUPTER. 395 



at resonance can easily be located within one or two per cent. I have 

 not been able to get quite such smooth resonance curves when the 

 spark-gap is employed as interrupter in the sending circuit. Experi- 

 ments are now in progress with commercial wireless telegraph circuits, 

 instead of the closed loops. 



6 10 12 1+ 16 18 



NO OF PLATES IN AIR CONDENSER 



Figure V. 



IV. Photographs with the Revolving Mirror. 



In the search for an explanation of the greater uniformity and 

 effectiveness of the mercury interrupter in producing inductive action 

 between circuits, I have taken a series of photographs of condenser 

 discharges through the interrupter. For this purpose the familiar 

 revolving-mirror apparatus was employed (Figure VI). The concave 

 mirror M has a focal length of 1.52 meters, and is driven by a battery 

 motor at a speed of twenty to seventy revolutions per second. The 

 interrupter, mirror, and plate are in a light-tight box, of which the end 

 carrying the photographic plate projects into a dark room. ^Neither side 



