12 



43.2 M. J. Jamin on the Theory of the Normal Magnet, and 



Thus the working capacities of telegraphic circuits may be in- 

 creased indefinitely by suitable arrangements. Practically, 

 however, it would seem that a limit would soon be reached, 

 from the rapidly increasing complication of adjustments required. 

 Besides, to keep them going, the telegraph-clerks must themselves 

 be electricians of a rather higher order than at present ; and, 

 considering the condition of the labour-market and the youth of 

 the school-boards, that would scarcely pay. Nevertheless, from 

 experiments I have made, I find it is not at all a difficult matter 

 to carry on four correspondences at the same time, namely two 

 in each direction ; and if we may suppose the growth of tele- 

 graphy will be as rapid in the future as it has been in the past, 

 it seems not improbable that multi-telegraphy will become an 

 established fact. 



In a following paper I intend giving the formulae necessary 

 for calculating the proper proportions of the resistances &c. to 

 suit different liues and apparatus, so that the greatest possible 

 amount of current may be driven through the receiving instru- 

 ments, where alone it is of practical service. 



April 19, 1873. 



LIV. On the Theory of the Normal Magnet, and the Means of aug- 

 menting indefinitely the Power of Magnets. By J. Jamin*. 



IN the sitting of the 16th December, 1872, 1 made known to 

 the Academy the process by which I was enabled to mea- 

 sure the force necessary to separate the same very small iron 

 contact placed on different parts of a magnet. This force is 

 measured in grammes ; I designate it by F. I will now state 

 how it varies for the different points in a magnetized plate which 

 is straight, long, flat, and broad. 



On the line drawn in the middle of the plate, parallel to its 

 length — that is to say, along the axis, F = 0, not only in the 

 centre, but to within a short distance of the two ends, after 

 which it increases rapidly as far as the extremities, where it 

 forms two equal curves which are convex in relation to the mag- 

 net, and of which the equation has been given by Biot. It is 

 the same on every line parallel to the axis, with this difference — 

 that the ordinates of the curves are greater towards the edges 

 than in the middle. I have studied only the axial curve ; it is 

 this which will be exclusively considered in what follows. 



For one and the same steel magnetized to saturation, F aug- 

 ments with the thickness of the plate, according to laws (pro- 



* Translated from the Comptes Rendus de VAcademie des Sciences, 

 March 31, 18/3. 



