SCIENCE AND PRACTICE. 



249 



glass tube g g. The mirror is raised or lowered in the coil 

 by turning the milled head of 

 the reel r, and may be removed 

 entirely from the galvanometer 

 after taking off the glass tube 

 g, through a slit in the frame 

 between the coils a a and a' d. 

 e is a glass plate to guard the 

 mirror from currents of air, and 

 d a solid cylindrical block of 

 copper put behind the mirror 

 for the purpose of retarding the 

 freedom of its oscillation, and 

 bringing it quickly to repose. 

 The checking action of a solid 

 mass of non-magnetic metal in 

 the presence of a moving magnet 

 has already been alluded to. 

 Arago believed this action to bo 

 due to the attraction and repuk 

 sion of currents of magneto- elec- 

 tricity set up in the mass by the 

 moving magnet, and which have 

 the effect of opposing its motion. 



22. Thomson's Mirror Galvanometer is a modification of 

 "Weber's, differing from it in many important regards. 

 Weber's instrument is admirable in fact, necessary for a 

 certain class of measurements ; but for others the mass and 

 sluggishness of the heavy steel mirror are objectionable, as 

 well as the distance of the galvanometer from the observer, 

 entailing as it does a length of connection wires which in 

 fine measurements may be found to be inconvenient. In 

 addition to this, the instruments, with all their adjuncts of 

 telescope, scale, illuminators, &c., are expensive and cum- 

 bersome. Professor Thomson has avoided all this in taking 

 a mirror whose weight does not exceed a few grains, and 

 whose momentum is therefore very small, and in dispensing 

 with the telescope by throwing a spot of light directly upon 



Fig. 128. 



