12 M. MELLONI ON THE FREE TRANSMISSION' 



it is by no means necessary to review in succession all the degrees that 

 contribute to the formation of this arc. Tiie application of our method 

 to the angles of 20° and 10° will be quite sufficient. This being done, 

 we shaH find an equal (juantity between their difference and the effect 

 produced by the simultaneous action of the moving forces. In other 

 words, let us produce a deviation of 20° to the right and one of 10° to 

 the left : let us then simultaneously expose the two opposite faces of the 

 pile to the two I'adiations which produce these galvanometric indica- 

 tions: the index will move to the right, and stop precisely at 10°. 

 Hence we infer that the force necessary to make the needle describe 

 the arc comprised between 10° and 20° is equal to the force required 

 to make it pass over the first ten degrees of the scale. Thus the pro- 

 portion of the degrees to the forces is perceptible as far as the 20th de- 

 gree on each side of zero. 



This fact seems opposed to the inference which might have been 

 made in examining the nature of the galvanometric action ; for, in the 

 successive rotation of the astatic system, the poles of tlie magnetic 

 needles depart from the mean line of the electric currents. The inten- 

 sity of the repulsive forces, therefore, decreases in proportion as the 

 angle of deviation increases. Whence we should conclude that the 

 effort necessary to make the needles exceed a given arc should change 

 as soon as the first degrees of the scale are passed. This would un- 

 doubtedly take place if all the electric currents lay in a vertical plane 

 passing through the line marked 0° ; but the circumvolutions of the me- 

 tallic wire which is wound on the frame placed under the graduated 

 circle are distributed to a certain extent on each side of this plain. In 

 the galvanometer which I have employed in my experiments, they cover 

 the two opposite arcs of 76°, the chords of which are perpendicular to 

 the line marked 0°. Thus so long as the oscillations take place within 

 certain limits there will always be electric currents situated on each side 

 of the needles. Now when the intensity of these currents is extremely 

 feeble, their sensible effect on the needles must cease at a very short 

 distance. Let us suppose this distance to be 1 8° of the division of the 

 galvanometer intended to show the degrees of electric action which 

 cause the deviations to the riglit and left for the first 20° of the scale. 

 These degrees of action must be extremely feeble in a very delicate gal- 

 vanometer. If, during these oscillations, the system of the needles is 

 confined within the two initial arcs of 20°, it is clear that it will always 

 be subject to the same action, whatever may be the position in which it 

 is placed ; for there Avill always be near its plane a series of currents 

 extending to 1 8° on each side, even when the system will occupy the 

 extreme limits. Tiie influence of the currents that are further distant 

 will, according to our hypothesis, be nothing. As the moving force 

 will therefore have a constant value, wc shall have to consider only the 



