1875.] 44:1 [Barker. 



used by Gauss in 1827, aud adopted subsequently by Poggeudorff and by 

 Weber, wbicb consists in attaching a mirror to the needle. By this 

 means, a beam of light may be reflected to the zero point of a distant 

 scale, and any deflection of the needle made clearly evident. The advan- 

 tages of this method are : — 1st, the motion of the needle may be indefi- 

 nitely magnified by increasing the distance of the scale, and this without 

 impairing the delicacy of the instrument ; and 2d, the angular deflection 

 of the needle is doubled by the reflection. These unquestioned advan- 

 tages have led to the adoption of this method of reading in the most 

 excellent galvanometers of Sir William Thomson. While therefore, for 

 purposes of research, this method seems to leave very little to be desired, 

 yet for purposes of lecture demonstration it has never come into very 

 great favor ; perhai>s because the adjustments are somewhat tedious to 

 make, and because, when made, the motion to the right or left of a 

 spot of light upon a screen fails of its full signiflcance to an average 

 audience. 



Another plan is that used by Mr. Tyndall in the lectures which he gave 

 in this country. In principle, it is identical with that employed in the 

 megascope; i. e., a graduated circle over which the needle moves is 

 strongly illuminated with the electric light, and then by means of a lens 

 a magnified image of both circle and needle is formed on the screen. The 

 insufficient illumination given in this way, and the somewhat awkward 

 arrangement of the apparatus required, have prevented its general 

 adoption. 



A much more satisfactory arrangement was described by Professor 

 Mayer in 1872,* in which lie appears to have made use, for the first time, 

 of the excellent so-called vertical lantern in galvanometry. Upon the 

 horizontal plane face of the condensing lens of this vertical lantern, 

 Mayer places a delicately balanced magnetic needle, and on each side of 

 the lens, separated by a distance equal to its diameter, is a flat spiral of 

 square copper wire, the axis of these spirals passing through the point of 

 suspension of the needle. A graduated circle is drawn or photographed 

 on the glass beneath the needle, and the image of this, together with that 

 of the needle itself, is projected on the screen, enlarged to any desirable 

 extent. The defect of this apparatus, so excellent in many respects, 

 seems to have been its want of delicacy ; for in the same paper the use of 

 a flat narrow coil, wound lengthwise about the needle, is recommended 

 as better for thermal currents. Moreover, a year later, in 1873, f 

 Mayer described another galvanometer improvement, entirely different 

 in its character. In this latter instrument, the ordinary astatic galvan- 

 ometer of Melloni was made use of, an inverted scale being drawn on the 

 inside of the shade, in front of which traversed an index in the form of a 

 small acute rhomb, attached to a balanced arm transverse to the axis of 

 suspension of the needle, and moving with it. The scale and index were 

 placed in front of the condensing lenses of an ordinary lantern, and their 



*American Journal of Science, III, iii, 414, June 1872. 

 tAmerican Journal of Science, III, v, 270, April, 1873. 



