SCIENCE AND PRACTICE. 



305 



For this measurement by Messrs. Siemens' testing-board, 

 the key K (Fig. 143) reposes upon the back- contact, by 

 which the battery is kept out of the circuit, the plug of the 

 triangular commutator U 3 is put into 

 the hole I-TII, introducing the unit 

 element into the branch a c, the holes 

 of r and p are stoppered, and r'( = 

 10,000) left open, the stoppers L L' 

 and L' E, are put in, and lastly the 

 adjustable resistance in the side b d 

 is made according to (3, equal to 



E _ g (10,000 + r") 

 990,000 (r" + g) 



The deflection of the needle is 

 obtained on the same side of zero 

 as in the measurement which this constant is to be used for 

 reducing, by means of the commutator u 2 . It is the expression 

 of the current due to the unit of electro-motive force and 

 unit of resistance. 



50. Measurement of Electro-motive Force. The method of 

 measuring how many times the 

 electro-motive force of the unit 

 cell is contained in the electro- 

 motive force of the battery em- 

 ployed, is the compensation sys- 

 tem of Professor Poggendorff, of 

 Berlin. The side a d, Fig. 144, is 

 opened by removing the plugs I* I/ I 

 and L' E, and a c is completed with 

 the unit element. Three circuits 

 parallel to each other are, therefore, 

 formed, combining in the points c 

 and b, viz. : 1) c f> b } 2) c a G b, and 3) 

 c K R b. u x is stoppered so that the 

 currents of the battery and constant 

 cell oppose each other in the gal- 

 vanometer circuit ; the key is kept 

 down ; of the resistance scales, r = 0, 



x 



Fig. 144. 



= 10, or 100, 



