OF RADIANT HEAT THROUGH DIFFERENT BODIES. 1 .'{ 



modifications which the active part of this force is made to undergo by 

 the different inclination of the needles to the direction of the currents ; 

 and these modifications are quite analogous to those which take place 

 in the portion of gravity that acts on the pendulum in dift'erent arcs of 

 oscillation. 



Now the force necessaiy to make the pendulum vibrate from one in- 

 clination to the other, is proportional to the difference of the cosines of 

 the angles which the two directions form with the vertical. Whence it 

 is clear that it remains sensibly constant in the arcs that are not far re^ 

 moved from the line of rest. The same effect must therefore be pro- 

 duced in the galvanometer also ; or, in other words, the force required 

 in this apparatus to increase the deviation of the index by a degree will 

 be constant near the line of zero, as is shown by experiment. 



From what we have just said it will be easy to see that the relation 

 between the degrees of the galvanometer and the forces which cause 

 the deviations of the needles, must depend on the sensibility of the 

 astatic system and the distribution of the wire on the frame *. It will 

 vary, therefore, according to the construction of the instrument, but 

 may be always determined by the method Me have mentioned. 



Experiment having shown that in my galvanometer the proportion of 



• In order to understand this clearly, it is sufficient to suppose a galvanome- 

 ter in which the circumvolutions of the wire are move numerous towards the 

 extremities than towards the central part. It is evident that under the action 

 of such a system the forces which produce the deviations, instead of increasing 

 or being merely proportional in the arcs near zero, must decrease as we approach 

 the extremities of the frame, in order to increase afterwards when the index has 

 passed these positions. 



As to the influence of the sensibility of the astatic system, we shall be able to 

 form a tolerably exact idea of it, if we imagine a galvanometer with the two 

 needles possessing very different degrees of magnetism. Then the terrestrial 

 globe will very powerfully affect both combined ; and, in order to produce the 

 least deviations, electric currents must be employed possessing much greater 

 force than those required to produce small deviations in a more perfect astatic 

 system. In the positions near zero, the electro-magnetic action produced by 

 the most distant currents, that is, the action of the currents situated at the ex- 

 tremities of the frame, will possess an energy sufficient at least to overcome the 

 resistance arising from the twisting of the suspension thread and the inertia of 

 the astatic system. It will therefore always contribute to move the oscillating 

 mass. Hence it is evident that if the needles are disjjlaced in the slightest de- 

 gree, the consequence will be a loss in the moving force ; for if the system ap- 

 proaches a certain arc at a certain extremity, it recedes at the same time double 

 the distance from the opposite extremity. Now we have already seen that, in 

 delicate galvanometers, the moving force is constant when the angles are small ; 

 and we have assigned the cause of this fact upon the incontestible principle that, 

 in small deviations of the instniment, the action of the currents situated towards 

 the extremities of the frame must be disregarded, not indeed because they have 

 no value, but because it becomes, in consequence of its distance, extremely 

 feeble, and incapable of surmounting the obstacles opposed to it by the torsion 

 of the silk thread and the inertia of the needles. 



