6 Mr Rudge, A self-recordiiKj electi'uineter, etc. 



tubing covered over with wire gauze, so as to prevent loss of radium 

 by rubbing, etc.; Avhilst allowing it to take up the potential of the 

 air. The collecting plate was supported at the end of an insulated 

 wire, and at such a height above the ground as would give a 

 deflection suitable to the sensibility of the electrometer. 



Up to the present time the apparatus has been used for the 

 purpose of taking records of the variations in the potential 

 gradient, due to the presence of clouds of dust raised by traffic on 

 the roads, or to the variation caused by the steam escaping from 

 passing trains. A number of representative curves are given. 



No. 1. This is a twelve hour record, taken at a station on the 

 Gog Magog Hill about four miles fi'om Cambridge, and so far from 

 the railway and roadway that traffic had no disturbing influence. 



No. 2 is a simultaneous record taken in Hills Road at a distance 

 of less than a quarter of a mile from the railway, so that every 

 passing train shows its influence in increasing the positive potential. 



Nos. 3 and 4 are a pair of simultaneous hour records, three 

 being taken at Cherryhinton reservoir, and four at about 300 yards 

 from the railway. The " peaks " in the latter indicate the passing 

 of a train. 



No. 5 is a one hour record taken on Hills Road and shows the 

 remarkable influence of the dust raised by passing vehicles. Every 

 vehicle, even an ordinary bicycle, if it raises dust, disturbs the 

 normal electrification. Nos. 6 and 7 are simultaneous records 

 taken at some little distance from the road. Nos. 8 and 9 were 

 taken near the "Long" road railway crossing and show the 

 influence of passing trains. Nos. 10 and 11 are a pair of simul- 

 taneous records, 10 being taken in the Railway yard, and showing 

 the effect of passing train and "shunting " operations ; 11 was taken 

 about a mile away from the line. 



All the potentials indicated are positive and the records are 

 reduced in reproduction, but an equal range of negative potentials 

 could be recorded, as only one half of the width of the photo- 

 graphic paper was used in the records given. The sensibility of 

 the instrument may be changed by varying the number of cells of 

 the charging battery. 



