178 



Prof. A. Schuster's Electrical Notes. 



If we further put g = np, where p is equal to the best galva- 

 nometer- resistance, 



s J ~ n + 1 



V 



n 



<x 



With a galvanometer of resistance np the sensitiveness is 



2 ^n 



therefore z times that of the obtainable maximum. And 



n-\- 1 



the reduction is the same whether the galvanometer -resistance 



is np or p/n. The following table giving the values of 



2 vn , r, i 



may be useful. 



n+1 



n. 



2 V w 



11. 



2 V^ 

 n+1 



1 



1 

 •943 



•866 

 •800 

 •745 

 •700 

 •661 

 •629 

 •600 

 •575 



11 



•553 

 •533 

 •515 

 •499 

 •484 

 •471 

 •458 

 •447 

 •436 

 •426 



2 



12 



3*.....'....... 



13 



4 



14 



5 



15 



6 



16 



7 



17 



8 



18 



9 



19... 



10 



20 







It is seen that if the galvanometer-resistance is five times too 

 great or too small, the sensitiveness is reduced by 25 per cent. 

 Except for very special purposes, such a reduction is not 

 of great importance ; but I think sufficient consideration is 

 not paid, in ordinary laboratory practice, to the proper choice 

 of galvanometer-resistance. The tendency is to use a galva- 

 nometer of too great resistance, probably because the makers 

 take more trouble with the more expensive instruments, and 

 in consequence they are more easily adjusted and less liable 

 to get out of order. With the same instrument it is con- 

 venient to be able to vary the resistance ; and this can be 

 done by using the pattern of a differential galvanometer, and 

 by dividing each of the two coils again into two parts. The 

 resistance may be varied in that case in the ratio of 1 to 16. 

 If we had the command of three galvanometers each having 

 four coils respectively of '01, 4, and 1000 ohms, we could 

 measure resistances varying between "0005 and 20,000 ohms, 

 and always be within 25 per cent, of the greatest obtainable 

 sensitiveness so far as the galvanometer is concerned. If.we 

 are only able to have one galvanometer, we can obtain a 

 useful combination by choosing one of four coils of about 



