40.7 OPTICAL PROJECTION 



happen to lie in or near the plane of the magnetic meridian, 

 it will be necessary to counteract the * dipping-needle ' effect 

 by adjusting a bar-magnet somewhere near, preferably under- 

 neath the table. 



The most sensitive and precisely calibrated astatic galvano- 

 meters are chiefly needed for very feeble thermo-pile currents, 

 as in investigating the heat of the spectrum in its colder 

 regions, or calorific interference bands (see Chapter XXIII.). 

 Such a galvanometer is easily projected in the vertical manner, 

 provided the base and dial-plate are of glass. 



The lecture galvanometers most generally used, however, 

 where comparatively but not excessively small currents are 

 to be demonstrated, are of the reflecting mirror class. For 

 ordinary experiments, one of the dead-beat types is most 

 convenient ; but I am only here concerned with the projection, 

 which is capable of a much better result than is generally 

 seen. A dim and very nebulous spot on the scale seems to 

 satisfy most demonstrators, and is perhaps sufficient for a 

 small audience. But it is very much better to use a small 

 lime-light lantern for the index-pencil, and, omitting any 

 lens in the galvanometer itself, to employ the 'focussed 

 parallel beam ' (p. 277) from the pencil attachment shown 

 in fig. 95. An aperture and lens should be chosen which, 

 focussing on the scale, gives a sharp and brilliant disc over 

 its whole width (about a quarter of an inch is a good average), 

 and this should be crossed by two wires arranged X fashion. 

 The great superiority of this method will not fail to be appre- 

 ciated when once seen. 



It is quite needless to describe in detail how every form of 

 current induction, either by other currents or by magnets, is 

 demonstrated by the galvanometer. 



241. Mutual Action of Currents and Magnets. The 

 general rule to be observed in devising apparatus to project 

 the wide range of phenomena discovered by Arago, Ampere, 

 apd Faraday, is always to select the most simple arrangement. 



