262 To project Galvanometer-deflections on a Screen. 



with the calcium-light it is distinctly visible in a room consi- 

 derably illuminated by daylight. With less illumination of 

 the room I have used the instrument when the calcium-light 

 was replaced by a kerosene-flame. 



Evidently the precision of the indications of the apparatus 

 just described are vitiated by the parallax of the index; for it 

 does not describe a cylinder which is an extension of the one on 

 which are drawn the graduations. This error is avoided by 

 cementing on the inside of the shade a curved piece of glass 

 whose radius of curvature equals the arm carrying the index, 

 and whose centre coincides with the axis of the aluminium wire. 

 With this modification in the apparatus I have succeeded in 

 reading with precision deflections to .6' of arc. 



By the following arrangement, deflections to 1' in an "arc 

 extending 5° on each side of the O-point can be determined. A 

 thin slip of microscope-cover glass is coated with a layer of 

 black varnish, and through this varnish are cut, in a dividing- 

 engine, fine equidistant lines. The diamond-shaped pointer is 

 replaced by a light piece of cover-glass, also coated with var- 

 nish, and having cut on it one fine vertical line. These lines 

 are illuminated by the lantern ; and in front of them is placed 

 an inch or an inch-and-a-half objective. On the screen we 

 have the graduations as a series of bright lines on a dark 

 ground, and along them moves the bright index-line of the 

 pointer. 



The zero-points of the scales can be brought accurately to 

 coincide with the normal position of the index by revolving the 

 shade on its base ; and by turning the transverse wire so that 

 it points towards the screen when the needles of the galvano- 

 nometer have come to rest, we can readily project the image 

 of the index and scale in any desired direction. 



Although there are some advantages in having the scales at- 

 tached to the galvanometer and in obtaining on the screen 

 their magnified images, yet we can save much time in the con- 

 struction of the apparatus by substituting for them scales drawn 

 directly on the screen in very black india-ink. 



My experience with this instrument has led me to prefer 

 the use of only one magnetic needle, the one enclosed in the 

 coil of the galvanometer ; and this needle I render more or less 

 astatic by means of a damping-magnet placed above the galva- 

 nometer and sliding on a vertical rod and rotating on its centre 

 around the same. By means of the magnet, one can with expe- 

 dition adapt the sensitiveness of the instrument to the require- 

 ments of special experiments; and thus the galvanometer is 

 admirably suited for ail experiments on radiant heat, electricity, 

 or magneto-electricity. In fact, on holding my hand at a dis- 



