404- Sig. Nobili and Antinori on the Electro-motive Force 



motions are made with the same rapidity, and with relation to 

 the same points of the magnet, the deviations are in the inverse 

 directions to each other, but precisely of the same value. The 

 detachments are, however, always equally instantaneous, and 

 for constancy of effect are preferable to approximations ; for 

 the latter to be always equally successful would require a 

 mechanical arrangement, which it is not worth while either to 

 imagine or to execute. By taking care that the lifter is con- 

 stantly in its right place and position, there will always be 

 produced the same deviation of the galvanometer when it is 

 detached from the magnet. This we repeat is a valuable re- 

 sult applicable in numerous cases, and perhaps proper to mea- 

 sure the force of large magnets in a more exact manner than 

 by the ordinary mode of ascertaining the weights sustained. 



The arrangement described is highly advantageous ; but 

 does it produce the maximum of electro-dynamic effects? 

 There is indeed another much better {Exp. Res. 46 note), which 

 consists in applying the electro-dynamic spiral to the central 

 part of the lifter, corresponding to the interval which separates 

 the poles of the horse-shoe magnet. In this position a spiral 

 of a few turns is able to surpass the effects of a far greater num- 

 ber of spirals otherwise disposed. Behold then the arrangement 

 which it is convenient to make to obtain all the effects of a 

 magnet. The central part of the lifter is to be entirely covered 

 with wire, leaving exposed only the extremities, which are to 

 come in contact with the pole of the magnet. The ordinary 

 form of the lifter is not the most convenient upon which to 

 arrange this species of large electro-dynamic ring, but upon 

 conveniently modifying its shape the wire may be applied with 

 facility, and thus the effect be obtained at its highest degree of 

 intensity. The reason is evident; for two conditions in fact 

 require to be fulfilled : one, that the spiral should be subjected 

 to all the influence of the magnetic force ; the other, that this 

 influence should be abstracted in the shortest possible time. 

 Now the wire round the lifter is exactly in the most favourable 

 position for the magnetic force to be concentrated upon it; 

 and this force vanishes the instant the lifter is detached, as is 

 required by the second condition. 



Spirals of various Metals {Exp. Res. 132. 139. 193. 208. S)-c.). 



The metals with which we have experimented are four, — 

 copper, iron, bismuth, and antimony : iron is interesting as the 

 foremost amongst magnetic metals(iEjr/:'./^(75.8.9.21 1.); bismuth 

 and antimony for the distinct position they hold in the thermo- 

 magnetic scale. In experiments made under circumstances 

 approximating to equality, it appeared that copper was the 



most 



