PIERCE. — THE COOPER HEWITT MERCURY INTERRUPTER. 391 



now employed also by Marconi and by Slaby, the arrangement at the 

 sending station is essentially the same as in the Tesla coil, with one of the 

 terminals, T, put to earth and the other, T 1} attached to one or more verti- 

 cal wires carried by masts. One object of this research is thus the attempt 

 to discover in what way and to what extent the Hewitt interrupter is 

 superior to the spark-gap in the wireless transmission of signals. 



The study of the mercury interrupter is also of interest in its relation 

 to the theories of electric conduction in gases and in its relation to the 

 phenomena of electro-luminescence. 



The present paper comprises : 



II. Quantitative measurements of the induction between circuits with 

 the two forms of interrupter in the sending circuit. 



III. Resonance between such circuits. 



IV. Photographs of the oscillations in the Hewitt interrupter with the 

 aid of the revolving mirror. 



V. Photographs showing the rapidity of recovery of the Hewitt 

 interrupter. 



VI. Calorimetric measurement of the ohmic resistance of the Hewitt 

 interrupter. 



VII. Determination of the proper vacuum for the Hewitt interrupter. 



II. Quantitative Measurement of the Induction 

 between Loops. 



In order to obtain a direct comparison of the mercury interrupter with 

 the spark in air between solid metallic terminals, I have measured the 

 intensity of signals obtained in a receiving circuit with the two forms of 

 interrupter respectively in the sending circuit. For a receiving instru- 

 ment recourse was had to a form of oscillating current galvanometer 

 devised by Fleming and employed in 1897 by Northrup, Pierce, and 

 Reichmann * in an experiment on induction between distant circuits. 

 Figure III is a diagram of this instrument. In the centre of the figure, 

 between S and N, is a carefully insulated coil usually of about one hundred 

 turns of fine wire. The coil has an internal diameter of about 1 cm., 

 and is put in series with the receiving circuit by means of binding posts 

 outside of the enclosing vulcanite box. For the purpose of varying 

 the sensitiveness of the instrument, the coil maybe removed and replaced 

 by another of any desired number of turns. Within the coil is suspended 

 a thin circular disc of silver foil about 6 mm. in diameter, to which a 



* Electrical World, Dec. US and 25, 1807. 



