230 



DISCHARGE OF ELECTRICITY. 



coating of the Leyden jar. The knobs of this air-break ought to be 

 kept brightly polished. The loop C is connected to eartli. The dis- 

 charge-tubes, which were in general either rectangular tubes or spher- 

 ical bulbs, where placed close to the turns of C. When the difference of 

 potential between A and B is sufficiently large, a spark i)asses across 

 the air-break, and the electrical oscillations set up produce a large 

 electromotive force in the neighborhood of the coil, sufficient under 

 t^ivorable circumstances to cause a bright discharge to pass through the 

 vacuum-tubes. In some experiments tlie jars, at the suggestion of 

 Prof. Oliver Lodge, were connected up differently, and are represented 



(WimJiurst) 



earth 



earlli 



by (/i) in Fig. 1. Two jars were used, the outside coatings of which, A 

 and B, Avere connected by the wire containing the primary coil C, the 

 inside coating of the hrst jar was connected to one pole of the Wims- 

 hurst, that of tlie second to the other. With this method of arranging 

 the jars no air-s])ace is required, as the sparks pass between the ter- 

 minals of the machine, and the ]>olishing of these terminals is not nearly 

 so important as tliat of the knobs of the air-break in the arrange- 

 ment («). 



Before proceeding to describe the appearance prescmted by the dis- 

 charge, I will mention one or two imints which may i)r(>ve useful to any 

 one who wishes to rei)eat the experiments. According to my experi- 

 ence the discliarge is more easily obtained in bulbs than in square tubes, 

 and with a Wimshurst machine than with an inductic^n coil. If an in- 

 duction-coil is used a break which will transmit a large current ought 

 to be substituted for tlie ordinary vibrating one supplied with such in- 

 struments. It is essential to sncci'ss that the gas in the bulbs or tubes 

 shoidd be quite dry and at a suitable ])ressnre; there is a pressure at 

 which the brilliancy of the discharge is a maxinuim, and as in endeav- 

 oring to get at this piessure the exhaustion may be carried too far, it 

 is convenient to use a form of mercury pumy which will allow of the 

 easy admission of a little gas; the pattern which I have used and found 



