Vertical- Lantern Galvanometer. 435 



1827, and adopted subsequently by Poggendorff and by Weber, 

 which 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 advantages of this method are: — 1st, the motion 

 of the needle may be indefinitely magnified by increasing the 

 distance of the scale, and this without impairing the delicacy of 

 the instrument j and, 2nd, the angular deflection of the needle is 

 doubled by the reflection. These unquestioned advantages have 

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

 excellent galvanometers of Sir William Thomson. While, there- 

 fore, for purposes of research this method seems to leave very 

 little to be desired, yet for the purpose of lecture demonstration 

 it has never come into very great favour — perhaps 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 significance to an average audience. 



Another plan is that used by Prof. 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 illumi- 

 nation given in this way, and the somewhat awkward arrangement 

 of the apparatus required, have prevented itsgeneral adoption. 



A much more satisfactory arrangement was described by 

 Professor Mayer in 1872*, in which he 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 con- 

 densing 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 

 1873f, Mayer described another galvanometer improvement, 

 entirely different in its character. In this latter instrument, 

 the ordinary astatic galvanometer of Melloni was made use of, 



* American Journal of Science, S. 3. vol. iii. p. 414, June 1872; Phil. 

 Mag. S. 4. vol. xliv. p. 25. 

 t American Journal of Science, S. 3. vol. v. p. 270, April 1873. 



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