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



upper a south pole. This is a phaenomenon of position always 

 ccciirring in the same direction with this kind of iron, which is 

 as incapable of retaining the magnetism received, as it is dispo- 

 sed to receive the new magnetism to which it may be subjected. 

 In our latitudes the inclination of the needle is about 63°. 

 The paper tube with its spiral was therefore arranged in that di- 

 rection, and an iron cylinder introduced; whilst in the act of 

 introducing it, the galvanometer was seen to move {Exp. Res. 

 146.), owing to the presence of an electric current excited by 

 the magnetism. On taking out the cylinder the motion was re- 

 versed : there is no doubt, therefore, that terrestrial magnetism 

 is sufficient of itself to develope currents of electricity. It should 

 not be concealed here, that in the above experiment the elec- 

 tricity is developed by the intermedium of soft iron intro- 

 duced into the spiral : this without doubt is true, but it is also 

 true that it is not essentially necessary to recur to this aid 

 to obtain unequivocal signs of the influence of which we 

 speak. On placing our cylindrical spiral so that its axis should 

 be parallel to the magnetic dip, and then inverting it by a 

 half revolution in the magnetic meridian {Exp. Res. 148.), we 

 observed at the comparative galvanometer the signs of a cur- 

 rent excited in the spiral by the sole influence of terrestrial 

 magnetism. 



It is not even necessary for this effect to place the spiral in 

 the direction of the dip : the experiment will succeed in the 

 vertical position; the effect is less, but always so distinct as 

 to remove every error {Exp. Res. 153, &.C.). 



We experimented with three copper wires of different 

 diameters, the smallest was 05, the second 0*66, and the third 

 1" millimetre in diameter. The effects increased with the size: — 

 the first gave deviations from 2 to 4; the second from 4 to 8 ; 

 and the third from 10 to 20. To obtain these great motions, 

 we operated in the usual way of inverting the current at the 

 most favourable moment, which is easily learned by repeating 

 the experiment a few times. 



In the present state of science this is most certainly the sim- 

 plest mode of obtaining the current * ; all is done by terres- 

 trial magnetism, which is everywhere. We purpose hereafter 

 to study the manner of increasing the effect, and of making 

 some useful applications, if certain apparatus which we pur- 

 pose constructing should meet our wishes {Exp. Res. 147. 154, 

 &c.). The first thought is that of using it to measure the ter- 

 restrial magnetic intensity; but what precision the mode may 

 be capable of, remains at present to be determined. 



[* A much more simple mode is described in ray paper at (170, &c.); for 

 neither spiral nor soft iron is necessary. — M. F.] 



The 



