78 Messrs. Ayrton, Mather, and Sumpner 



translation along their own magnetic axes, the magnetic 

 needles being drawn into, or pushed out of, coils acting 

 like solenoids. 



3. Galvanometers in which the magnetic system is stationary, 



and the moving system consists of a coil turning about 

 an axis on its own plane. 



4. Instruments based on the heating of a conductor by the 



passage of a current through it. 



Type 1 includes tangent-galvanometers, Thomson's reflecting- 

 galvanometers — in fact, so many well-known instruments that 

 further description is unnecessary. In addition to galvano- 

 meters with straight magnetic needles, it may be noticed that 



Fig. 9. 



type 1 also includes instruments containing a bell-shaped 

 horseshoe-magnet; since the bell-shaped magnet is only a 

 device for producing the equivalent of a very powerful and 

 permanent short flat magnet. 



