on Galvanometers. 73 



artificially dried by sulphuric acid, is due to Messrs. Eidsforth 

 and Mudford. To prevent any possible overflowing of the 

 sulphuric-acid vessel, a syphon passes through the base on the 

 Tantalus cup principle, and which empties the liquid when it 

 rises to the level of the bend of the syphon. 



YI. Proportionality of Deflexion and Current. 



Some authorities are of opinion that it is of comparatively 

 little consequence what is the law connecting deflexion and 

 current, since for very accurate work any galvanometer must 

 be carefully calibrated experimentally, and the exact law 

 connecting deflexion and current thus ascertained. But, on 

 the contrary, although the necessity of calibration for accurate 

 work is perfectly true, experience has shown us that for 

 ordinary practical work much time is saved if the readings on 

 the scale are approximately, even if not quite accurately, pro- 

 portional to the currents producing them. This latter result 

 can, of course, be obtained either by fitting the lengths of the 

 divisions on the scale to the peculiarities of the galvanometer, 

 or by starting wdth a uniformly divided scale and constructing 

 the galvanometer in such a way that equal additions to the 

 current produce equal angular deflexions. 



In certain cases, when, for example, the galvanometer is 

 mainly employed for measuring currents all having about one 

 definite value, as for example in the case of a voltmeter used 

 on an electric-lighting circuit, it is obviously desirable 

 to have the scale widely extended at the part where it is 

 most used, and to obtain this result it is better to have 

 the divisions crow r ded together elsew T here. But, since the 

 power of accurately subdividing the spaces on a scale by eye 

 is much increased if all the spaces be of equal length, it is 

 preferable, in the case of galvanometers for general use, and 

 when all parts of the scale are equally valuable, to judiciously 

 construct the galvanometer in such a way that the whole scale 

 may be divided into equal distances which are directly pro- 

 portional to equal increments of current, than to give the 

 coils and needle a shape arrived at in a haphazard fashion, and 

 then attempt to experimentally subdivide the scale to suit the 

 vagaries of the galvanometer. 



In a former paper communicated by Prof. Perry and one of 

 the authors to this Society, it was pointed out that while a 

 galvanometer maybe a " proportional " instrument if the zero 

 for no current be that corresponding with the needles being 

 in a symmetrical position to the coils, the same instrument 

 may be very far from proportional if the zero for no current 



