ELECTRICAL MEASUREMENTS. 285 



movement is damped, just as if it were swinging in oil or 

 some resisting fluid. Here is a circular plate with a 

 groove cut in it, in which a conducting wire can be placed, 

 and the diameter of the circle can be accurately measured. 

 The distance of the circle from the magnet is also 

 accurately measured by means of a divided scale. Here 

 we have again the means of obtaining a conductor of 

 accurately known length in an accurately known position 

 relatively to the magnet, and the deflection of the magnet 

 can be observed. The amount of deflection would be 

 ascertained in this case, not by a graduated circle, but 

 by means of a mirror which is attached to the magnet and 

 turns with it, and the deflection of the mirror is obtained by 

 means of a telescope. There is another apparatus here on 

 the same principle. If we place a divided scale opposite 

 the mirror, the divisions of the scale are reflected from the 

 mirror back into the telescope which is placed opposite. 

 In this way an extremely small deflection of the mirror, and, 

 therefore, of the magnet, can be detected. The method 

 of reading these two instruments is the same, but the first 

 is an absolute instrument and the second is merely a com- 

 parative apparatus. It is a very convenient form of its 

 kind. There are three pairs of coils belonging to it, one a 

 pair of long coils of thin wire, and the other pairs being 

 shorter and of thicker wire. There is the same arrange- 

 ment as in the first I showed you for varying the distance 

 between the conductor and the magnet. 



Here again are other forms of galvanometers, but still 

 comparative instruments only. These are different forms 

 of Sir William Thomson's Reflecting Galvanometer, 

 perhaps one of the most delicate instruments of the kind 

 ever constructed to indicate the passage of an exceedingly 

 small current. It will give us the comparative measure- 

 ment of very weak currents, but not an absolute measure. 



Here, again, is an apparatus, constructed by Professor 

 Guthrie, which will also give us comparative measurements. 

 It depends upon the mutual attraction or repulsion of little 

 bits of iron magnetized by the passage of a current. 



So much for the various ways in which electro-magnetic 

 action can be made to serve as a basis for a system of 

 measurement. It is much the most frequently employed 

 principle, but we can also employ electro-dynamic action 



