280 



A. TANAKADATE 



section. One portion of this was put into the jaw, the remaining 

 portions being a« far removed as possible from the instrument. • A 

 current from 20 Daniell cells was passed through the ribbon ; a 

 standard tangent galvanometer and a resistance box being in the 

 circuit. Simultaneous readings of the standard galvanometer and the 

 pocket instrument were taken for different strengths of current, 

 obtained by varying the resistance. From these the value of the cur- 

 rent corresponding to 1 division of the pocket instrument was easily 

 calculated. The following table gives the results of the co?n})arison 

 thus made with the working apparatus. 



Staiulard Galvanometer. 



Pocket Galvanometer. 



Eeadiug. 



Kednced to 

 Ampci-es. 



lieadiug. 



Value of 1 Divi- 

 sion in Amperes. 



12.0 

 19.7 

 28.9 

 35.5 



.0350 

 .0575 

 .0844 

 .1037 



12.4 

 20.6 



39.5 

 37.4 



.565 

 .558 

 .554 

 .555 



Mean .558 



As the instrument was graduated to 100 divisions we could 

 measure with this up to 56 ampères, and, as will be seen later, by 

 placing the wire below the instrument up to 168 (= 3 x 56) amperes. 

 In making the comparison, we must place the standard galvanometer 

 at a considerable distance from the ribbon, which becomes a strong- 

 magnetic shell when the current is passing. In fict it is best to 

 put the two instruments on different tables, and make two series 

 of observât ionS; one with the current direct and the other with it 

 reversed. For measuring a moderate current such as 1 or 2 amperes 

 we can advantageously repeat the circidt three or four times by simply 

 coiling the conductor so as to make a temporary ribbon like that 

 just described. 



