424 Use of the term "Resistance " in Physical Phenomena. 



which we are thus led, in place of Lamont's formula above 

 given. Prof. S. P. Thompson has shown that this formula, 

 which is the same as that known as Frolich's, corresponds 

 with the facts better than Lamont's, 



In what precedes I have, for convenience in quotation, 

 used the ordinary term magnetizing force, and regarded it as 

 a cause. As, however, so-called magnetizing force is identical 

 in dimensions and physical nature with the rest of the mag- 

 netic induction which it develops, I use the expression in 

 this way under protest only; for I regard it as inconceivable 

 that cause and effect should be of identical nature, unless the 

 effect reacts again as a cause, so that the smallest original 

 cause drives the effect up to saturation. Without going into 

 detail I may mention an analogy where cause and effect are 

 identical in nature, and the process does therefore necessarily 

 always go to saturation ; viz., the case of the multiplication 

 of germs, in a habitat of limited capacity. Here the indivi- 

 duals which formed the original cause multiply ; the progeny 

 constitute at once the effect and an increase of the cause. And 

 the smallest original infection of the cause is enough to 

 develop saturation, i. e. the highest population that the habitat 

 can maintain. Now in the case of magnetism, magnetizing 

 force and the rest of the magnetic induction are by definition 

 identical in nature. If they are cause and effect, the effect 

 should therefore act again as cause, so as to produce satura- 

 tion from the smallest force ; but this does not occur. I 

 prefer therefore to regard the difference of potential as the 

 cause, as is always done in the case of the electric current. 



There is only one further point. It may be maintained 

 that the mention of Ohm's law involves sub silentio the 

 assumption that the corresponding law holds true, i. e. that 

 the resistance is constant. If this is supposed, it is only 

 necessary to explain that it is not intended. It is clearly 

 legitimate to measure a quantity in the manner suggested by 

 Ohm's law (i. e. by the ratio of cause to effect), and speak of the 

 laws or courses of values thus obtained as representing in 

 the different cases the analogues of Ohm's law. These are 

 the laws which occupy the place in the various matters dealt 

 with, which Ohm's law occupies in the subject of the flow of 

 electricity, i.e. they express the ratio of cause to effect in 

 the different cases. But the statement that they are the 

 analogues of Ohm's law does not involve the position that 

 they are identical with it, any more than the definitions of 

 Potential or Force in electricity and magnetism are identical 

 with those in mechanics or with each other. 



The main points I have dealt with are : — That resistance 



