Force of the Electric Discharge through Gases. 431 



The exhaustion in most of the experiments was made with 

 an oil air-pump capable of producing a vacuum of less than 

 1 mm. of mercury. 



Two large coils of thick copper wire, capable of carrying a 

 current of ten or twelve amperes for a short time, were used 

 for producing the magnetic force to deflect the discharge. 

 These coils were wound on wooden bobbins with 150 turns in 

 each coil, and with a mean radius of 17 cms. each. They were 

 fixed in a frame with their planes parallel, and perpendicular 

 to the line joining their centres, at a distance apart of 17 cms., 

 the mean radius of either coil, — Helmholtz's arrangement for 

 producing a uniform field about the middle point of the axis 

 of the coils. 



The current in the coils was given by an ammeter in series 

 with them. 



The discharge was produced by a Ruhmkorff's coil, and the 

 mean current in the discharge was measured by a galvano- 

 meter in the discharge-circuit. 



In the experiments the discharge-tube was placed so that 

 the middle point of the discharge w T as as near the middle 

 point of the axis of the coils as possible, and with the direction 

 of the discharge at right angles to that axis. 



Prelimina ry Experiments. 



Some preliminary experiments were made with different 

 gases in the discharge-tube, and for a given magnetic force, 

 the deflexion of the discharge produced in each gas was 

 observed. As the deflexions could not be very accurately 

 compared with the eye alone, a camera was employed ; it was 

 permanentlv fixed and focused on the discharge so that a 

 photograph could be obtained at any time by simply uncapping 

 the lens. In order to compare the deflexions produced by a 

 constant magnetic force in the different gases, the mean 

 current in the discharge, as measured by the galvanometer, 

 was kept constant by varying the pressure of the gas in the 

 discharge- tube. 



The following table gives the gases used, arranged in order 

 of the magnitude of the deflexions obtained in them by the 

 method stated above : the deflexion in hydrogen was least. 



(1) H. ( N. (4) NO. (6) NH 2 . 



(2) CI? W Ui, (5){cl- (7){gfo 



The position of the gases marked thus ? in the above table 

 is rather doubtful, as the oil-pump could not be used for 



2 H2 



