APPENDIX TO EXPERIMENTAL SECTION. 



263 



open. 



Place the rider against the bridge wire a few cms from 



its extremity a and turn the 

 commutator into the position in 

 which the G deflection is smallest, 

 then move the rider J until a 

 balance is obtained, e.g., the spot 

 of light on the G scale stands at 

 Zero. This will be very near the 



FIG. 57. E non-polar- \ e ft en( j. This fraction a of the 

 isable electrodes. C **.* 4-1 



commutators. G gal- wire compared with its total 



V* C ^^l^pVbrMge length gives the E.M.F. of the 



bar. J rider, a and b tissue, &c., in terms of a Daniell 

 on the bridge wire, the length of which is . 



1000mm. (I'l volt )- 



As 1000 is to a J so is 1-1 (volts) to x = volts of the tissue. 



Measurement of resistance with the bridge box. When 

 c and d are in the same proportion as a and b there is no deflection 

 of the galvanometer. This condition is sought for and is known as 

 the zero method. 



B L 



* T * f a:0::c:c 



- G 



FIG. 58. Diagram of Paul's resistance box and Wheatstone's bridge. P Shows 

 the connections of the terminals in the actual box and W the same transferred to 

 the usual diagram of Wheatstone's bridge. B Battery. G Galvanometer. L Line 

 or the points to which the unknown resistance X is to be connected. B Spring 

 key which on closure leads to the proportional arms, r to R the variable resistance 

 R arranged in four rows ; the movable plugs p are connected for zero or no 

 resistance in diagram P. 



