ON THE COOPER HEWITT MERCURY INTERRUPTER. 



By George W. Pierce. 



Presented by John Trowbridge June 17, 1903. Received January 6, 1904. 



I. Introduction. 



Mr. Peter Cooper Hewitt has devised a new form of interrupter 

 designed to take the place of the spark-gap in the wireless transmission 

 of signals and in the production of high potential discharges with the 

 Tesla transformer. Mr. Hewitt's interrupter employs the discharge 

 between mercury electrodes in an exhausted bulb instead of the usual 

 spark in air between solid metallic terminals. 

 Figure I is a diagram of the usual form of Cooper- 

 Hewitt interrupter. At the bottom of an exhausted 

 bulb 15 or 20 cm. in diameter are two deep de- 

 pressions containing pools of mercury, between 

 which the discharge is made to pass. Short pieces 

 of platinum wire (1.5 mm. in diameter) fused 

 into the glass serve to lead the current into the 

 bulb. To prevent unequal heating of the sealed- 

 in wires the two protuberances may be dipped into 

 tumblers of mercury to which the connections are Figure I. 



made. 



In the preparation of the interrupters employed in these experiments 

 the mercury for the bulbs was carefully purified by distillation and by 

 washing through a nitric-acid filter ; when the mercury is slightly con- 

 taminated the interrupter soon becomes inoperative. The bulbs were 

 attached to a condenser and the source of current while being pumped, 

 and their vacuum was tested from time to time during the exhaustion. 

 In these tests care must be taken not to send the discharge through the 

 interrupter when it contains too much air, as oxygen under the action of 

 the discharge contaminates the mercury. When the vacuum was too low, 

 the discharge through the bulb showed striations, and when the vacuum 

 was too high, the discharge could not readily be started. The proper 

 vacuum could be distinguished by the appearance of the bulb or by the 



