Screen the Deflections of the Needles of a Galvanometer. 261 



inclined mirror placed above them he obtains their images on 

 the screen. 



In the Number of this Journal for July, 1872, 1 published the 

 description of a new form of "Lantern-Galvanometer," which 

 article was subsequently republished in Dr. Carl's Repertorium. 

 This instrument, although it served admirably in the experiments 

 for which it was specially devised, yet is of difficult construction 

 and of limited application when compared with the very simple 

 apparatus I will now proceed to describe. 



G is the glass shade of the galvanometer, on which, at g, 

 are drawn in india-ink the vertical graduation-lines of the in- 

 strument. A is a piece of aluminium wire, to whose lower 



end are fixed the needles of the galvanometer, and whose upper 

 end is perforated with a small hole, so that the system can 

 be suspended by a silk fibre. A fine wire of German silver 

 (W) is attached transversely to the aluminium wire, and has its 

 ends bent downwards at right angles to its length. This trans- 

 verse wire can be placed at any azimuth by rotating it around its 

 centre, which is coiled two or three times round the vertical wire 

 of aluminium. On one of the bent ends of the transverse wire is 

 cemented a diamond-shaped piece of light paper or foil ; and the 

 other end carries a small ball of wax whose weight equals that of 

 the piece of paper or foil. The diamond courses around the 

 shade at about 1 millimetre from its interior surface, with its 

 lower point just above the lines of graduation. At C are re- 

 presented the condensing-lenses of an oxy hydrogen lantern 

 whose jet and lime are at L. is the objective, which gives on 

 the screen the magnified image of pointer and scale, as seen 

 at S. This scale is not graduated into equal angular divisions, 

 but its units represent units of dellecting-force traversing the 

 galvanometer; and this scale is therefore derived from a careful 

 calibration of the instrument. 



The sharpness of the image on the screen is admirable ; and 



