[PATTERSON] SELF RECORDING ELECTROMETER 25 



vided the boom and its counterpoise were the same as before. Now 

 as the boom and counterpoise are the chief factors in limiting the 

 sensitivenesss of the instrument, it might be assumed that it would 

 have been much simpler to have increased the size of the needle and 

 obtained the same result in that way. The preliminary experiments 

 indicated that the 6^ boom was the best. Having used the best 

 length of boom and made the diameter of the quadrants as great as 

 possible for this length of boom, the best arrangement of boom and 

 needle was obtained. Any further increase in the needle would have 

 necessitated a longer boom and owing to the additional weight would 

 have required a stronger suspension, thus largely counteracting any 

 gain obtained by the increased size of the needle. There remained 

 therefore the method, which was adopted, of double decking the 

 quadrants and using a two vaned needle, to obtain the greater 

 sensitiveness. 



Another question that arises is as to whether an instrument that 

 requires a battery of a few volts can be kept as constant as a battery 

 of 100 volts or more. In this instrument 12 volts P. D. across the 

 quadrants is sufficient for measuring the potential of the air; a 

 reference to the figure will show that a change of 1% in the voltage 

 will cause a change of about 1% in the readings of the instrument. A 

 change of -5 volt in a battery of 12 volts would be large when no 

 current is being used, yet this would not alter the value of the 

 readings by more than 4%. 



The advantages claimed for the instrument are: — 

 I. Simplicity of construction. 

 II. Little attention required when in working condition. 



III. High degree of sensibility, enabling the potential of the 

 air to be measured with a battery of only a few volts. 



IV. Uniformity of scale over the range of the recording paper. 



Sec. Ill, 1913—2 



