— 60 — 



Since the present trial aims to determine the resistance of wood to 

 electricity in general, no particular choice of voltage was considered 

 necessary, so an ordinary direct current of 110 volts was applied. 



IV. Measurement of Resistance 



In the present measurements, the arrangement was as shown in the 

 figure. The instruments used , were a 

 D'Arsonval's Galvanometer, a Shunt, a 

 Eesistance box, a Change-over-key and 

 Battery. Thus, d°, to be read by the 

 inclination of the galvanometer. 



Let resistance of galvanometer be G, 

 resistance of the shunt S, resistance of 

 resistance-box R, motive force of the 

 battery E, its inner resistance b, and 



constant of the galvanometer K (by constant K we mean the circuit 

 resistance necessary to produce the inclination of one grade on galvano- 

 meter by the voltage applied) . We obtain : — 



d° : 



E 



R + -„"^+b 



G-l-S 



= 1°: 



E 



K 



G+S 



K=d (R + 



GS 



G + S 



+b) 



GH-S 



S 



Let n indicate multiplying factor of the shunt. So long as the resistances 



of the galvanometer and of the inner part of the battery are small compared 



with the other resistance, the two last values in the bracket can practically 



G + S 

 be neglected, so we obtain K=dRn, n= — J-^ 



In place of resistance-box, the test specimen X will be connected 

 by change-over-key, to read the inclination of the galvanometer. Then 

 the factor of the shunt will be known as ni, and angle of inclination di, 

 resistance needed x. We obtain: — 



x=- 



K 



dini 



d, 



R 



ni 



