BAIN AND ITS OKIGIN IN THUNDERSTORMS. 383 



During the course of the experiments two additions were made to the apparatus 

 just described, which it had not been possible to get ready before the first few storms 

 occurred. 



A second Benndorf electrometer L was added to record the potential gradient. 

 It is almost impossible to get a satisfactory method of recording the potential 

 gradient during thunderstorms, because the changes in the gradient take place 

 more rapidly than the instruments used can follow them, and the range over which 

 the potential gradient varies is extreme. It was therefore considered sufficient to 

 record only the predominant sign of the potential gradient during each two minutes' 

 interval. This was done by putting an insulated bamboo rod MM, having an 

 umbrella rib attached to its end, through a window into the open air, and connecting 

 it to the second Benndorf electrometer.* It was also arranged that this simple 

 collector should be connected to earth at the end of each two minutes. The same 

 current which recorded on the first Benndorf electrometer was used for the second, 

 and the records were thus simultaneously taken. 



The second addition consisted of an arrangement to record the lightning 

 discharges. A long wire was carried from the open air into the hut and connected to 

 a coherer fitted with the usual decohering device. The current of the latter passed 

 through the circuit of the " time marker " on the second Benndorf electrometer, 

 so that each lightning discharge was recorded by dots on the edges of the paper 

 which was receiving the record of the potential gradient. The coherer was 

 purposely made somewhat insensitive in order not to record distant discharges, 

 and on this account it often missed weak near discharges. The device, therefore, 

 could not be said to count the number of discharges, but it gave a good idea as to 

 whether the rainstorms were accompanied by many or few electrical discharges. 



During the monsoon, for weeks together, the humidity of the air practically 

 remained at 100 per cent., and thus everything, both indoors and outdoors, became 

 very damp. Under such conditions, provision had to be made for keeping the 

 insulation of the instruments in good condition. This was done by arranging that 

 all insulators should be within the case FF, which was kept warm by continuously 

 burning a lamp under the lower end of a pipe, 7 cm. in diameter, passing through 

 the case in the manner shown in fig. 1. This precaution prevented all difficulty 

 with the insulation of the instruments ; but another difficulty connected with 

 insulation could not be entirely avoided. This was caused by spiders spinning their 

 webs from the insulated parts of the system to earth-connected objects. Since the 

 collector of the potential gradient apparatus had to be exposed in the open air, it 



* The rod carried a small tube containing radium at its end, but it is not believed that this materially 

 assisted the equalisation of potential between the rod and the surrounding air, the ordinary " dissipation " 

 at the end of the rod being sufficient for the purpose. The capacity of the rod being small, the quantity 

 of electricity accumulated at the end of the rod did not in the author's opinion disturb sensibly the 

 lines of force in the neighbourhood of the rod. [April 14, 1909.] 



