1896.] 



Mr Moncfanan, Discharge in Vacuo, etc. 



217 



The multiple arc tube A was joined to a mercury gauge B 

 and onward through the clip H to the receiver of a mercury 

 pump. By this arrangement the pressure may be regulated to the 



Fig. l. 



greatest nicety by keeping the receiver exhausted and carefully 

 opening H, a little ah' may be drawn out. And with a little 

 practice there is no difficulty in going through a whole series of 

 readings varying two or three millimetres at a time. 



The first series of experiments were tried with ordinary air, 

 and it was found that when the pressure was reduced to 19 mm. the 

 reversal of the spark commenced, but it ceased when the pressure 

 fell to 11 mm., beyond which the spark passed down one side only. 



Change of gases. I next tried the effect of changing the gas, 

 and found that only one refused to produce any reversal while 

 another acted very poorly, all the other gases giving reversals at 

 pressures varying with the nature of the gas used. 



Very great care was exercised in preparing pure and dry N, H, 

 and O, the object being to test whether a pure dry elementary gas 

 would produce the phenomenon ; also pure N was mixed with HC1 

 (pure) with what result will appear in the table following. 



It is interesting to notice that the two gases that produce the 

 least effect are those that were found by Faraday to give the worst 

 examples of brush discharge. 



Density of spark. There is a particular density of spark more 

 favourable than others for producing the reversal. The most 

 convenient adjustment of the vibrating contact breaker of the coil 

 can be found only by trial, as it varies with the coil used and with 

 the tube. I found that a position about half-way between the 

 position of brightest spark and that at which the hammer vibrated 

 with frequent stoppages, answered very well with the particular 

 instruments used. The proper density appears to be as small a 

 discharge as can be got to pass steadily, since a dense one bursts 

 through and often passes down both sides at once. 



Variation of the electrodes. In the experiments described the 

 electrodes were platinum wires, on the ends of which small knobs 



VOL. IX. PT. IV. 



17 



