on Galvanometers. 63 



the coil, and that the mirror (which is, of course, employed 

 to reflect a ray of light, and not, as its position inside the coil 

 would seem to infer, to be acted on electromagnetically) 

 should be placed outside the coil, or between the upper and 

 lower sets of coils when two pair of coils are employed. A 

 specimen of a galvanometer constructed in this way and con- 

 taining certain other improvements, which wiITT)e referred to 

 later on, was submitted some years ago by Mr. Mudford to 

 Sir William Thomson, who expressed approval of the devices 

 employed, and, since that time, many galvanometers differing 

 in other details, but all embodying the principle of not wasting 

 valuable space in the coils, have been constructed for the 

 Central Institution. 



It is interesting to notice that the common method of placing 

 the mirror inside the coil is an irrational survival of an old 

 custom kept up, like the two buttons at the back of a coat, 

 when its use is almost forgotten. Gauss and Weber put their 

 mirror inside the coil because the mirror was made of polished 

 steel and was also the magnetic needle. Sir William Thomson 

 put the mirror inside the coil in his " speaking galvanometers," 

 for receiving messages on submarine cables, because he desired 

 to render the space near the mirror as air-tight as possible in 

 order to obtain great damping. And the ordinary maker of 

 astatic galvanometers puts the mirror inside the coil, because 

 he has seen it there in other galvanometers, and it has never 

 occurred to him to put it anywhere else. 



If the ray of light has not to go into the coil no hole need 

 be left and the coil may be wound practically to the centre. 

 It is necessary, however, that the space left unwound inside 

 the coil should be somewhat larger than is required simply to 

 allow the needle to turn through a small angle, because if the 

 windings were carried to the axis of the coil in the immediate 

 neighbourhood of the needle the effect on the latter of the 

 current passing through the innermost coils would be opposed 

 to that of the rest of the coil. To find the shape and volume 

 of the least space that should be left unwound we proceeded 

 as follows : — 



Let A (fig. 3) be the radius of a convolution of wire whose 

 plane is perpendicular to the paper, the section of the convo- 

 lution being represented by the small circle at A, then, from 

 Maxwell's ' Elect, and Mag/ vol. ii. plate xviii., the lines of 

 force due to a current passing round this convolution are as 

 indicated. Therefore, if a magnet whose half length is NP be 

 placed at a distance ON from the plane of the convolution 

 such that NP is a tangent to the line of force at P, no torque 

 will be exerted on the magnet by a current passing round the 



