116 PROCEEDINGS OF THE AMERICAN ACADEMY. 



to procure insulated copper or silver wire for the suspended coil so free 

 from paramagnetic properties that the coil shall not have a permanent 

 " set " in the field, too strong to be conveniently controlled by the tor- 

 sion of the gimp through which the current enters the coil. In the 

 case of a quantometer where there is practically no controlling moment 

 from the suspending fibre, the paramagnetic properties of the coil may 

 be very troublesome ; and in some of the most recent instruments the 

 angular movements of the coils, due to given changes of induction 

 through the turns of the exploring coils, are somewhat different ac- 

 cording as the movement is towards the left or towards the right. If 

 a telescope and scale be set up in such a position that the behavior of 

 the coil can be watched after it has moved through a considerable angle, 

 urged by a sudden, definite change of flux in the exploring coil, it will 

 often be found that the coil does not remain even approximately at 

 rest, but moves steadily and so rapidly that a considerable error is 

 introduced if the given change of flux through the exploring coil is 

 made slowly. It is desirable, therefore, to test an instrument of this 

 kind carefully before using it. 



If great accuracy is not required, a good fluxmeter, of some standard 

 make, and of sensitiveness suited to the work to be done, is, in experi- 

 enced hands, a most useful instrument ; the time needed to establish a, 

 current of given strength in the coil of a large electromagnet with a 

 solid core may be several minutes, but a very good fluxmeter will, 

 nevertheless, show directly, with an error of not more than 2 per 

 cent, the change of magnetic flux through the core. 



If the fluxmeter coil is not wound on a closed metal frame, the 

 mutual damping effect of currents in the coil and in the core which 

 it surrounds are not always effective unless the resistance of the ex- 

 ternal circuit, made up of the exploring coil and its leads, is fairly small 

 compared with the resistance of the suspended coil itself An instru- 

 ment, therefore, which works very well with an exploring coil of a small 

 number of turns often becomes quite useless when, in order to get the 

 required sensitiveness, the observer tries to employ an exploring coil 

 made of many turns of fine wire. On the other hand, if a fluxmeter of 

 this kind is too sensitive for a given piece of work, it is not always easy 

 to reduce the sensitiveness quickly. 



If the flux changes to be measured are large, it is often convenient 

 to have a fluxmeter the coil of which consists of a few turns either 

 wound on a copper frame or else accompanied by several turns of stout 

 wire closed on themselves. It is possible to use such an instrument 

 with many different exploring coils and to change its sensitiveness 

 within wide limits by varying the resistance of the external circuit. 



