Atmospheric-Electric Observations, 1915-16 



383 



not very desirable in the case of the Einthoven electroscope, or he must make a fresh cah- 

 bration curve each day. Again, the computational work involved in the construction and 

 use of such curves is considerable when much work has to be done. For these reasons the 

 general principle has been adopted of always noting the time taken by the fiber of the 

 electroscope in passing between two fixed readings on the scale of the electroscope. The 

 electroscope system is in each case connected to a potentiometer system used in connection 

 with a voltmeter, so that the electroscopes may be charged to any desired potential. On the 

 completion of the main observations the fixed readings referred to are reproduced, by chai'g- 

 ing the electroscope with the potentiometer, and the corresponding readings in volts are 

 read off from the voltmeter. This throws the constancy of indications on the voltmeter, an 

 instrument which, both as regards the stage of its development and the nature of its 



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Fig. 21. — Wiring Scheme for the Atmospheric-Electric House on the Carnegie. 



design, is such as to maintain a higher degree of constancy than is required in atmospheric- 

 electric work. The voltmeter used is a minature Weston instrument, with ranges of 3 

 volts and 150 volts, and the potentiometer system is obtained by utihzing an adjustable 

 resistance. Theoretically one potentiometer system would be sufficient for all the electro- 

 scopes; but in view of the difficulty of even standing still when taking observations in 

 a heavy sea, it is desirable to put the matter of convenience of manipulation in the fore- 

 ground, and a separate potentiometer system is attached to the base of each of the four 

 electroscope systems in use in the observatory. Only the voltmeter is common to all the 

 systems, and this is fixed to the wall in a position convenient for observation from all parts 

 of the room. 



Figure 21 gives a diagrammatic sketch of the wiring arrangements for the potentiometer 

 systems. A, B, C, and D are the electroscopes for the conductivity apparatus, ion-counter. 



