830 Prof. J. J. Thomson on the Discharge of Electricity 



Appearance of the Discharge. 



Let us suppose that we have either a square tube placed 

 outside a square primary or a bulb placed inside a circular 

 coil of wire, and that we gradually exhaust the discharge- 

 tube, the jars sparking all the time. At first nothing at all is 

 to be seen in the secondary, but when the exhaustion has 

 proceeded until the pressure has fallen to a millimetre or 

 thereabouts, a thin thread of reddish light is seen to go round 

 the tube situated near to but not touching the side of the 

 tube turned towards the primary. As the exhaustion pro- 

 ceeds still further, the brightness of this thread rapidly in- 

 creases, as well as its thickness; it also changes its colour, 

 losing its red tinge and becoming white. On continuing the 

 exhaustion the luminosity attains a maximum, and the dis- 

 charge passes as an exceedingly bright and well-defined ring. 

 On continuing the exhaustion, the luminosity begins to di- 

 minish until, when an exceedingly good vacuum is reached, 

 no discharge at all passes. The pressure at which the lumi- 

 nosity is a maximum is very much less than that at which 

 the electric strength of the gas is a minimum in a tube pro- 

 vided with electrodes and comparable in size to the bulb. 

 The pressure at which the discharge stops is exceedingly low, 

 and it requires long continued pumping to reach this stage. 

 We see from these results that the difficulty which is ex- 

 perienced in getting the discharge to pass through an ordi- 

 nary vacuum-tube when the pressure is very low is not 

 altogether due to the difficulty of getting the electricity from 

 the electrodes into the gas, but that it also occurs in tubes 

 without electrodes, though in this case the critical pressure is 

 very much lower than when there are electrodes. In other 

 words, we see that as the state of the bulb approaches that of 

 a perfect vacuum its insulating pow T er becomes stronger and 

 stronger. This result is confirmed by several other experi- 

 ments of a different kind which will be described later. 



The discharge presents a perfectly continuous appearance, 

 with no sign of striation, of which I have never observed any 

 trace on any of these discharges, though I must have ob- 

 served many thousands of them under widely different con- 

 ditions. 



Action of a Magnet on the Discharge. 



The discharges which take place in these tubes and bulbs 

 are produced by periodic currents, so that the discharges 

 themselves are periodic, and the luminosity is produced by 

 currents passing in opposite directions. As this is the case, 



