on Galvanometers. 71 



even removed without detaching the connecting wires from 

 the coils. The galvanometer shown in fig. 6 is, when in use, 

 covered up by the brass case supporting the controlling 

 magnet, illustrated in fig. 1. 



Hinged boxes containing the coils of a reflecting-galvano- 

 meter appear to have been employed even earlier than this by 

 Mr. Pepper, at the Polytechnic, but that form of construction 

 was, apparently, in 1868 unknown to, or at any rate unused 

 by, instrument makers. 



In the case of galvanometers intended to be highly insulated, 

 these hinged boxes may w T ith great advantage be made of 

 ebonite, and to prevent the boxes becoming electrified and 

 acting on the suspended needle electrostatically, one terminal 

 of the galvanometer should be joined to the outside brass case. 



In the galvanometer shown in fig. 6, the boxes are kept 

 closed by a screw which is screwed into the framework of the 

 instrument, but in the galvanometer shown in fig. 7 this 

 screw is replaced by a spring button, which is, a more con- 

 venient arrangement. 



Y. Insulation of Coils and Terminals. 



In the ordinary method of constructing reflecting-gal- 

 vanometers the wires are brought under the base to terminals 

 fastened on to it. But such an arrangement is most un- 

 satisfactory, since, no matter how thoroughly the upper 

 part of the ebonite base may be cleaned and dried, leakage is 

 almost sure to take place between the two wires, between the 

 terminals, and between both the wires and the terminals to the 

 ground along the inaccessible under surface of the ebonite 

 base. In fact, when one of the authors was engaged, on be- 

 half of the Indian Government, in 1872 testing telegraph 

 insulators at Messrs. Siemens's works at Charlton, it was the 

 common practice every morning to light a gas-burner near 

 the galvanometer to dry it, and to diminish the leakage to a 

 workable amount. And it appears to the authors that the form 

 of expensive astatic reflecting-galvanometer, that is still made 

 by some firms, with the wires underneath the base, is but a sur- 

 vival of the antiquated type of leaky electrostatical apparatus, 

 to use which required that the whole air of the room should 

 be first dried. Fig. 6 shows the obvious way of overcoming 

 this difficulty, and which consists in supporting the frame- 

 work and the coils of the apparatus as well as the terminals on 

 paraffined corrugated ebonite pillars p, which are maintained 

 clean and dry by their being kept entirely inside the brass 

 cover of the instrument Q (fig. 1). The terminals T at the 



