246 



Professor J. J. Tliomson 



[April 13, 



produce nearly so large an effect as the exhausted bulb : this might 

 be due, as we have seen, to the sulphuric acid being either too good 

 or too Dad a conductor. I can show that it is the latter by putting a 

 bulb in filled with a stronger solution, which has a higher conduc- 

 tivity than the weak solution ; if the smallness of the effect produced 

 by the weak acid were due to its being a better conductor than the gas, 

 then increasing the conductivity would still further diminish the effect 

 of the acid, you see, on the contrary, that the strong acid produces a 

 distinctly greater effect than the weak, hence the rarefied gas in the 

 bulb is a better conductor even than the strong electrolyte. Let us 

 consider for a moment the molecular conductivities of the two sub- 

 stances, the rarefied gas and the electrolyte. The pressure of the gas 

 is about yj^ of a millimetre, while in the electrolyte there are sufficient 

 molecules of the acid to produce, if they 

 were in the gaseous state, a pressure of 

 more than 100 atmospheres ; thus the 

 conductivity of the gas estimated per 

 molecule is about 10 million times that 

 of the acid, this is greater than the mole- 

 cular conductivity of even the best con- 

 ducting metals. 



If the pressure of the gas is diminished 

 below a certain point, the conductivity 

 begins to diminish. I have here an ex- 

 periment which I hope will show this. 

 The apparatus (Fig. 5) consists of two 



bulbs, one outside tbe other; the inner 



bulb contains air at a low pressure, while 

 the space between the two bulbs is a 

 very high vacuum containing practically 

 nothing but a little mercury and its va- 

 pour. The amount of mercury vapour in 

 this space is, at the temperature of the 

 room, exceedingly small, but as the apparatus is heated the vapour 

 pressure increases, and we are thus able to produce a fairly wide range 

 of pressure in the space between the bulbs. The outer sphere is 

 surrounded by the coil connecting the outer coatings of the two 

 Leyden jars. "When the space between the bulbs is a conductor, 

 the alternating currents circulating in the coil will induce in this 

 conductor currents whose inductive effect is opposite to that of the 

 currents in the coil ; and in this case this layer will screen off from the 

 inner bulb the electromotive force due to the alternating currents in 

 the coil. If, on the other hand, the space between the bulbs is a non- 

 conductor, the inner bulb will be exposed to the full effect of these 

 forces. We now try the experiment : you observe that when the 

 mercury is cold, and consequently the pressure in the space between the 

 bulbs very low, a bright discharge passes through the inner bulb, 

 while the space between the bulbs remains quite dark ; when we heat 



