382 DR. G. C. SIMPSON ON THE ELECTRICITY OF 



each dot on the paper indicated the charge which the receiver had obtained from 

 the rain in two minutes. This earthing magnet could be disconnected when it was 

 desired to test the insulation of the system by the method described above. 



The amount of rainfall was recorded in the following way : The end of the metal 

 pipe EE which drained the water from the receiver ended within a vertical cylinder 

 J. From the end of the pipe the water fell in large drops, and since the drops 

 detached themselves from the pipe well within the cylinder they carried away no 

 electricity. The drops then fell into the funnel of the rain-gauge K. This was of 

 the ordinary tipping bucket pattern, recording on a drum which revolved in 24 hours. 

 One slight alteration was however made. Instead of allowing the pen to be always 

 touching the paper, it was only brought into "contact with it for a moment at the 

 end of each two minutes' interval, this being effected by means of an electromagnet 

 actuated by the current, which excited every two minutes the magnets already 

 referred to. As every dot on the rain-gauge trace corresponded with a dot on the 

 electricity trace, it was possible to correlate the two records and find how much rain 

 corresponded with each deflection of the electrometer. In order to make this 

 correlation quite certain the electrical circuit which actuated the " hour marker " of 

 the Benndorf electrometer was disconnected from the clock and arranged so that the 

 circuit was automatically closed for an instant each time the bucket tipped. Thus 

 every time the bucket tipped a dot was printed on each side of the electrometer 

 trace, and if the rain was only light, so that the bucket tipped only once in about 

 two minutes, the number of tips could be counted from the dots. If, however, the 

 rainfall was rapid these dots were not separated but formed a line ; the rainfall was 

 then measured for eacli two minutes' interval from the rain-gauge trace. 



This method of measuring the rainfall had one serious drawback. The rain was 

 only registered when the bucket tipped, and as this only took place when 0'014 cm. 

 of rain had fallen, the registration was not satisfactory with light rain. The 

 registration was also not satisfactory at the beginning of a shower, for in this case 

 the first tip did not t;ike place until after considerably more than 0'014 cm. of rain 

 had fallen, owing to a certain amount of water being used up in wetting the receiver 

 and the pipe through which the water passed from receiver to gauge. Nevertheless, 

 after the first tip and when the rain was not very light the method worked 

 admirably and gave practically no trouble in use. In fig. 1, for the sake of clearness, 

 the rain-gauge is shown out of place. In use it was behind the case FF, in which 

 position the tube EE was much shorter than it has been necessary to show it in the 

 diagram. 



The corrugated walls of the hut were carried, as shown in the figure, to. a height 

 of about two metres above the roof of the hut. This was done in order to protect 

 entirely the mouth of the receiving apparatus from the earth's electrical field, and 

 to prevent the wind from sweeping across the mouth of the cylinder CC, which 

 would have interfered with the entrance of the rain into the receiver. 



