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Calibration of Leaf. — The instrument can be calibrated by connecting 

 to known potentials and noting the deflections of the leaf. A storage battery 

 of three or four hundred volts is convenient. Readings should be taken for 

 every few volts from to the maximum and a curve plotted. X = deflection, 

 Y = volts. If a large voltage battery is not at hand a 110 volt D. C. circuit 

 can be used making connection to a resistance as in Fig. 5. The voltmeter, 

 V, should be read at the same time that the deflection of the leaf is read. A 

 calibration curve from to 110 volts can then be obtained. Fo'r the higher 

 points proceed as follows: Charge the leaf to maximum voltage by means 

 of a rubber rod. A body of small capacity, small compared to the capacity 

 of the electroscope, 1 or 2 cm., say, is mounted on an insulated handle. A 

 coin on a small rubber rod will answer. This is first grounded and then 

 touched to the charged system. The gold leaf falls. The capacity is re- 

 moved, grounded, and the position of the leaf noted. The operation is 

 repeated until the leaf falls to on the scale. 



If C is the capacity of the electroscope, and 

 c is the capacity of the coin, 

 Q, the quantity of electricity, 



Vi V2 is 1st, 2nd, potential of the leaf, 



di d 2 is 1st, 2nd, deflections of leaf, 



then, Qi = CVi = (C + c)V 2 

 Q 2 = CV 2 = (C + c)V 3 

 Q B = CV B = (C + c)V (B+1) 

 C + c V! V 2 V n 



C V 2 V 3 V (B + 1 , 



The last three or four deflections should be on the part of the scale 

 already calibrated. That is, the potentials should be less than 110 volts. 

 If V B and V B + t are known by comparing with d B and d M+ j on the calibration 

 curve. Since, 



V V 1 



T n v n — 1 



V«+i V B 



then, 



V n _i can be calculated. V n —\ being known, then V„_ 2 can be determined. 

 In like manner all Vs can be determined up to Vi. Knowing V and the cor- 

 responding deflection, d, the curve can be extended up to the maximum 

 deflection. 



