220 Royal Institution. 



moved along the beam until the latter was horizontal ; and that state 

 was ascertained by a double arm support, which sustained the beam 

 when out of use, brought it into a steady state when moving, and 

 delivered it into a condition of freedom when required. The motion 

 of the box to the right or left, so as to place the object in the middle 

 of the magnetic angle, was given by two of the screws before spoken 

 of; the motion to the given distance from c, by the other four. 



Supposing the distance from c towards m to be adjusted to 0*6 of 

 an inch, when the beam was loaded above, and no object before the 

 magnet (the beam having been of course previously adjusted to its 

 normal position, and the torsion-index placed at zero), it then 

 remained to determine the return of the beam to its place when the 

 object had been suspended on it and repelled : this was done in the 

 following manner. A small plane reflector is fixed on the beam, 

 near its middle part, under the point of suspension ; a small tele- 

 scope associated with a divided scale is placed about six feet from 

 the reflector, and in such a position that when the beam is in its 

 right place, a given degree in the scale coincides with the fine wire 

 in the telescope. Of course the scale appears to pass by the wire 

 as the beam itself moves, and with a double angular velocity, because 

 of the reflexion. As it is easy to read to the fiftieth and even to 

 the hundredth of an inch in this way, and as each degree occupies 

 apparently 2*4 inches with the radius of 6 feet, so an angular motion, 

 or difference of -j^-oth of a degree, could be observed ; and as the 

 radius of the arm of the beam carrying the object was 6 inches, such 

 a quantity there would be less than -5-^00^^^ ^^ ^^ i^ch ; i. e. the 

 return of the beam to its first or normal position by the torsion force 

 put on to counteract the repulsion, could be ascertained to within 

 that amount. When an object was put on the adjusted beam, if 

 diamagnetic it was repelled ; and then, as the observer sat at the 

 telescope, he, by means of a long handle, a wheel and pinion, put 

 on torsion until the place of the beam was restored ; and afterwards 

 the amount of torsion read off on the graduated scale became the 

 measure in degrees of the repulsive force exerted. At the time of 

 real observations, the magnet, balance and telescope, were all fixed 

 in a basement room, upon a stone floor. But it is unnecessary to 

 describe here the numerous precautions required in relation to the 

 time of an observation, the set of the suspension wire by a high 

 torsion, the possible electricity of the object or beam by touch, the 

 effect of feeble currents of air within the box, the shape of the object, 

 • the precaution against capillary action when fluids were employed 

 as media, and other circumstances ; or the use of certain stops, and 

 the mode of procedure in the cases of paramagnetic action; — the 

 object being at present to present only an intelligent view of the 

 principles of action. 



When a body is submitted to the power of a magnet, it is affected, 

 as to the result, not merely by the magnet, but also by the medium 

 surrounding it ; and even if that medium be changed for a vacuum, 

 the vacuum and the body still are in like relation to each other. In 

 fact the result is always differential ; any change in the medium 



