372 Prof. Ayrton and Mr. Mather on Galvanometers. 



Now for 1 division deflexion at a scale distance of 1000 

 divisions, #=2oVo> so taking 



r=10 seconds, 



p = l-64xl0- 6 , and 



o- = 50, 



corresponding with B = 5000 approximately, a high value for 

 short permanent magnets, we get 



6-32 



Current per 1 division = j-qwI 2 V/ + 0"1 C.G.S. units, 



6-32 



= -Jq9 /'V/ + 0-1 amperes . (17) 

 From this formula the following table has been calculated. 

 Table VII. 



Half length of needles (I) 

 in centimetres. 



Amperes per division. 



Divisions per 

 Microampere. 



005 



6-12 Xl0 -12 



163,000 



04 



2-86 x 10 ~ u 



35,000 



015 



7-16X10 -11 



14,000 



0-2 



1-38 xl0~ 10 



7,250 



03 



3-eoxio -10 



2,775 



0-4 



7-14xl0~ 10 



1,400 



0-5 



i 



1-22X10 -9 



820 



With these results before us it is interesting to examine how 

 nearly the limits here given have been approached in actual 

 instruments. 



One galvanometer for which the dimensions of the magnets 

 are known is that mentioned in line 9 of Table II. In this 

 instrument the magnets are 0*82 centimetre long, therefore 

 £ = 0*41. From Table VII. an instrument with such a needle 

 and wound to have a resistance of 1 ohm should give about 

 1350 divisions per microampere under the assumed conditions. 

 On referring to Table II., line 9, we find the value of D/r* (the 

 expression most suited for a high-resistance instrument with 

 small coils) actually obtained to be 380, or little more than a 



quarter I ~^ J the calculated possible value. 



