Magnetic Effect to Thickness of Wire with Insulation. 113 



Having obtained the value of r by means of either equation 

 (2) or (4), it is easy to obtain the resistance of the galvanometer 

 G in B.A. units by means of the following formula : — 



9-718 :G; 



therefore 



(•0005) 2 * ar*{r + s) 2 c " 

 000000202458 Im 



G= 



«r 2 (r-}-s) 2 c 



(5) 



Let in = 6 square inches, 

 / = 5" 



s = 0"'o00625*, 

 p= 0"-0005, 



c = 1, or pure annealed copper at 0° Centigrade, 

 a = 4. 



1. 



External 



resistance, 



in B.A. 



units. 



2. 



Value of \, 

 in inches. 



3. 



r, calcula- 

 ted by equa- 

 tion (2). 



4. 



r, calcula- 

 ted by equa- 

 tion (4). 



5. 



Resistance 

 of galvano- 

 meter, in 

 B.A. units. 



1000 



2000 



3000 



10000 



100000 



1234-82 



2469-64 



3704-46 



1234820 



12348-2-00 



: 00609l 

 •005099 

 004594 

 003364 

 001835 



! 006097 

 •005105 

 •004601 

 •003372 

 •001847 



907 



1782 



2641 



8432 



74345 



A few words now with regard to the practical application of 

 the theoretical results we have obtained may not perhaps be con- 

 sidered out of place. 



It is at once evident that a very large alteration in resistance 

 will be caused by even a very slight alteration in the size of the 

 wire ; it is therefore very important, when drawing wire for any 

 given purpose, to measure the diameter very accurately. This 

 may easily be done with a good screw- micrometer ; but the in- 

 strument that I have found most useful for this purpose may be 

 briefly described as follows (see fig. 4)-f\ 



Two straight edges, each exactly 10" long, are fastened together 

 at one end, and separated at the other end by the insertion be- 

 tween them of a small piece of wire exactly 0' ;, 1 in diameter. One 

 of the straight edges is graduated along its length into inches 



* This value of 5 was obtained by, the actual measurement of the thick- 

 ness of the silk on a sample of covered wire. 



t The divisions and numbers on the right-hand side show also the 

 diameters and numbers of the wires of the Birmingham wire-gauge, accord- 

 ing to Culley's ' Hand-book of Telegraphy.' 



Phil. Mag. S. 4. Vol. 39. No. 259. Feb. 1870. I 



