:^87 ] 



LIX. Remarks on Mr. Rainey's Theory of Magnetic Re- 

 action. By the Rev. William Ritchie, LL.D., F.R.S., 

 Professor of Natural Philosophy in the Royal Institution of 

 Great Britain and in the University of London. 



To the Editors of the Philosophical Magazine and Journal. 



Gentlemen, 



TF the explanation given by Mr. Rainey, with regard to the 

 reaction of the hfter, (p. 220,) be admitted, it will completely 

 overthrow the Newtonian law of the perfect equality oi action 

 and reaction. To save you the trouble of preparing a new cut, I 

 shall use the one employed by Mr Rainey in his last commu- 

 nication. Mr. Rainey takes for granted 

 that a piece of soft iron B, placed in the 

 direction of one of the sides of the horse- 

 shoe magnet, will have a magnetic power, 

 represented by b, induced on its remote 

 extremit}', whilst the power of the pole S 

 still retains its inducing power b. Now 

 the simplest experiment shows that S can- 

 not induce a power on a piece of soft 

 iron without having its own power dimi- 

 nished by an eqiml quantity. If, there- 

 fore, b represent the absolute power of 

 the pole S, the magnetism induced at the 

 other extremity of the soft iron must be less than b, the dimi- 

 nution depending on the length of B. Hence when brought 

 into the position across the poles, the pole S has not induced 

 the magnetism represented hy a -\- b, otherwise its 0W7i mag- 

 netism would have been completely destroyed. The s^me re- 

 mark applies to the pole N. Hence 2 a will not represent the 

 magnetism at the pole S, nor 2 b that at the pole N. There- 

 fore the contact of the lifter cannot induce the magnetism a + b 

 in the magnet A, whatever be its temper or state of magnetism 

 with regard to saturation. 



The example which Mr. Rainey gives of increasing the 

 magnetism of a weak magnet, which has been previously mag- 

 netized to saturation and has had its magnetism diminished, is 

 one of the most unfortunate examples he could have chosen. 

 I have formerly shown, in the Philosophical Magazine, that 

 if magnetism be induced in one direction and then destroyed, 

 the original magnetic state is easily restored. 



The reaction of the soft iron does not therefore create or 

 induce magnetism, it only assists by its reaction in restoring 

 in some measure the weak magnet to its original state. If 



