82 Messrs. Ayrton, Mather, and Sumpner 



windows have holes in them to allow the terminals T, with their 

 ebonite collars E, to be screwed into the brass terminal blocks, 

 t j t. At the front of the instrument there is a circular plate- 

 glass window G, to allow the light to pass through, and pro- 

 tected by a brass cap when not in use. 



Type 3 includes the well-known d'Arsonval galvanometers, 

 with the modification described to this Society some two years 

 ago for obtaining u invariable sensibility " by replacing the 

 ordinary torsional suspension with one having extremely small 

 torsional rigidity, and inserting a number of small magnets in 

 the moving coil, which, being attracted by the large stationary 

 magnet, produce the control. 



Type 4 we do not propose touching on in this paper, as that 

 type of instrument formed the subject of communications pre- 

 viously made to this Society by Prof. Perry and by one of the 

 authors on u Hot-Wire Voltmeters " and " Twisted Strip 

 Voltmeters." 



It is important that the results of the numerous tests on 

 many specimens of each of the first three types of galvano- 

 meter that have been carried out in the laboratories of the 

 Central Institution, as well as the tests that have been made 

 elsewhere, and the results of which we have collected together, 

 should be reduced to the same standard ; so that the relative 

 merits of the different types of galvanometer, as well as of the 

 different specimens of each type, should be able to be usefully 

 compared. A satisfactory standard of comparison, however, 

 cannot easily be arrived at ; and we are not aware that the 

 question of comparing galvanometers of totally different types 

 has previously received much attention. The " figure of 

 merit " of a galvanometer is sometimes defined as " the 

 amount of current which will produce one division or degree 

 of deflexion " *. But although the figure of merit, so defined, 

 tells us the sensibility of a particular galvanometer with a 

 particular adjustment of the controlling magnet, and with a 

 particular distance of the scale, it gives us no means of com- 

 paring this galvanometer with other forms, since this figure of 

 merit can be varied by altering the position of the controlling 

 magnet &c. — in fact, is not a constant of the particular galva- 

 nometer. Something much more definite than this is there- 

 fore required for our purpose. 



A little consideration shows that the sensibility of a galva- 

 nometer in actual practice is made up of many factors, some 

 of which depend on the skill of the experimenter himself. In 

 view of the sensibility of any particular galvanometer being a 



* < A Handbook of Electrical Testing,' by H. R. Kempe. 



