to be employed in Testing with Wheatstone's Diagram, 35 



hi Siemeiis's units. 



9=f 

 by equation (3). 



9 >=Jc(l-J/km 2 ). 



9-9'- 



100 



85-56 



83-90 



+ 1-66 



200 



164-00 



162-00 



+ 2-00 



300 





236-40 



+ 



500 



, , 



379-50 



+ 



700 



. , 



516-60 



+ 



900 





648-90 



+ 



1000 



762-00 



714-00 



+ 48-00 



m in this Table =0-0026; i. e. 

 8 =0-03 niillim., 

 A, =55 (pure copper at0 c ), 

 B=0'2 metre, 

 A = 200 square millims. and c=4, 



supposing the area A divided into squares. 



The above Table shows that when ?72 = - 0026, which it may 

 sometimes exceed, the corrected value for g differs between 

 14'44 per cent, and 23*8 per cent, from the corresponding external 

 resistance k, a difference which is evidently too great to be neg- 

 lected where we have to deal with weak electrical currents, for 

 instance in measuring resistances with Wheatstone's diagram. 



Now we will suppose we have to determine the best resistance 

 of the galvanometer in Wheatstone's diagram, then 



thus 



(a + d)(b + c) _ a + d 

 a+b+c+d ™~ c + d 



9 = 



a + d 

 c + d 



{>-</m^'} 



m 



the insulating covering of the wire being considered in this ex- 

 pression for g. If we put in formula (6) 



m = 



(no insulating covering), we have again our former expression (2), 



_ {a + d){b + c) _ a + d 

 9 ~ a+b+c+d ~ c + d 



c. 



I may mention here that, where the external resistance varies 

 between wide limits, it will be better to divide the given space 

 into two equal parts and fill each of them with wires of different 

 diameters, so as to use them either successively or in parallel 

 connexion. 



P2 



