324 



only very brief battery applications, it would be possible to work with 

 so high a rate of heating as that, without having the results much 

 vitiated by it. But ^ of a cell per foot will give only -0676 of 

 heating effect per second, and will be quite a sufficient battery power 

 to use in most cases. In the case we have supposed, for instance, 

 of conductors only three inches long, the electromotive force on each 

 would then be about ^WG f tne electromotive force of the cell. What 

 we denoted above by e and / in equations (7) would therefore each 

 have this value. Hence, by equation (4), we see that the effect of a 

 difference of YGGV between SS' and TT' would be to give q-p the 

 value T0 ' of the electromotive force of a single cell. Now one 

 of the light mirror* galvanometers, which I commonly use, re- 

 flecting the image of a gas or paraffine lamp to a scale 25 inches 

 distant, would, if made with a coil of 50 yards of copper wire of 

 moderate quality, weighing 5 grains per foot, give a deflection of 

 half a division of -fa of an inch on this scale, with an electromotive 

 force of T0 T - of a single cellf . Hence by using such a galvano- 

 meter, and primary and secondary conductors of sufficient resist- 

 ances to fulfil the condition of doing away with sensible error from 

 imperfect connexions in the manner explained above, but yet of 

 resistances either less than or not many times greater than the re- 

 sistance of the galvanometer coil, it is easy to test to y^^ the resist- 

 ance of a copper wire or bar not more than 3 inches long. The 



* The mirror is a circle of thin " microscope glass " about three- eighths of an 

 inch in diameter, silvered in the ordinary manner ; and a small piece of flat file 

 steel of equal length, attached to its back by lac varnish, constitues the " needle" 

 of the galvanometer. The whole weight of mirror and needle amounts to from 

 1 to 1 grain. It is suspended inside the galvanometer coil by single silk fibre 

 about inch long. It is necessary to try many mirrors thus prepared, each 

 with its magnet attached, before one is found giving a good enough image. I 

 am much indebted to Mr. White, optician, Glaegow, for the skill and patience 

 which he has applied to the very troublesome processes involved. 



t In this state of sensibility the needle is under Glasgow horizontal magnetic 

 force of the earth alone ; and, with its mirror, it makes a vibration one way in 

 about '7 of a second. In many uses of my form of mirror galvanometer, both 

 for telegraphic and for experimental purposes, I find it convenient to make its 

 indications still more rapid, though, of course, less sensitive, by increasing the 

 directive force by means of fixed steel magnets. On the other hand, I use fixed 

 steel magnets to diminish the earth's directing force and make the needle more 

 sensitive, when very high sensibility is wanted ; but this would be inconvenient 

 for the application described in the text, because effects of thermo-electric ac- 

 tion would be made too prominent. 



