RAIN AND ITS ORIGIN IN THUNDERSTORMS. 381 



20 cm. above the level of the roof and its lower end just within the hut. To prevent 

 the rain-water which struck the sides of this cylinder from running into the 

 receiver, a conical rim CO was soldered inside the bottom of the cylinder and the 

 water was drained away through the pipe D. This rim reduced the effective opening 

 through which the rain fell to a diameter of 29 cm. 



The receiver AA, which was placed immediately below the bottom of the cylinder, 

 was a galvanised iron vessel 50 '5 cm. in diameter and 31 cm. deep, having a 

 slightly rounded bottom so that the water which fell into it ran off through the pipe 

 EE. The receiver was supported on three legs, which passed freely through three 

 holes in the top of the case FF, each being insulated on a sulphur-coated ebonite 

 rod I fitting into a firm tripod fixed on to the bottom of the case. 



The potential of the receiver was recorded automatically every two minutes by 

 means of a Benndorf* self-registering electrometer G of the usual pattern, registering 

 in the following way : To the needle of a quadrant electrometer a long aluminium 

 boom is attached and swings freely over a strip of paper 12 cm. wide, which is slowly 

 moved forwards by means of a clock. Every two minutes the clock closes an electric 

 circuit which actuates a magnet and causes a bar to press the end of the boom 

 sharply into contact with the paper through a typewriter ink ribbon. In this way 

 the paper receives a series of dots each representing the position of the boom, i.e. 

 the deflection of the needle, at the instant the circuit was closed. In order to mark 

 the time a second circuit is closed each hour, and a magnetic is thus excited which 

 causes two dots to be imprinted, one on each side of the paper exactly in a line with 

 the end of the boom ; thus the line joining these dots is at right angles to the 

 length of the paper, and would pass through a dot made at the same time by the 

 boom. This device will be referred to later as the " hour marker." As shown in the 

 figure, the Benndorf electrometer was placed within the case FF. 



When the receiver was connected to the Benndorf electrometer and it received no 

 charge, a series of dots was printed on the paper in a straight line, but when a 

 charge was imparted to the receiver the needle of the electrometer was deflected and 

 with it the boom, so that the dot made at the end of the next two minutes' interval 

 indicated the amount of the deflection. If the instrument were then left to itself 

 and received no further charge, the record would show a series of dots in a curved 

 line inclined to the zero line. The inclination of this line was a measure of the rate 

 of leak from the charged system. 



For the purposes of measuring the charge brought down by the rain, an earthing 

 device H was brought into use. This consisted of a light earth -connected wire 

 which, by means of an electromagnet, could be brought into contact with the 

 receiver and connect it to earth. This magnet was excited by the current which 

 caused the registration of the electrometer, so that at the instant that the potential 

 of .the receiver was registered the latter was also connected to earth. In this way 



* BENNDORF, 'Phys. Zeit.,' vol. 7, p. 98, 1906. 



