422 Mr. Bosanqnet on the Use of the term "Resistance " 



definitions of the dimensions of Force different both from 

 that used in mechanics and from each other. 



It is fully understood in these cases that analogy is all 

 that is involved, and nobody supposes that there is any 

 question of identity. 



I will now for a moment compare the case of magnetic 

 resistance with a natural case of ordinary resistance, to show 

 that the analogy between the two is extremely close. 



Consider a mass of matter in any condition ; suppose it 

 enclosed in a vessel and subjected to compression. Then the 

 cause will be the pressure, the effect the compression, and the 

 resistance on the analogy of Ohm's law the ratio of pressure 

 to compression. If we choose to measure the compression 

 by the inverse of the volume, the resistance will be the pro- 

 duct of pressure and volume. 



If we suppose the matter to be at first gaseous, and the 

 compression to take place with loss of heat according to 

 Boyle and Marriotte's law, the resistance will be so far 

 constant. But as the compression advances, and the liquid 

 and solid states are approached, the resistance will necessarily 

 increase, and when the compression reaches a certain value 

 it will be practically incapable of proceeding further, and the 

 resistance will increase indefinitely with the pressure. The 

 resistance is therefore here a function of the compression, 

 i. e. of the effect, just as magnetic resistance is a function of 

 magnetic induction ; and, if we omit the initial part of the 

 curves of magnetic resistance of bars, the rest of them is 

 oddly similar to the course of the resistance in the case 

 imagined. 



Now the resistance in the case imagined (of the compres- 

 sion of matter) is not constant, but is a function of the effect 

 just as magnetic resistance is. Is this want of constancy 

 an objection to the use of the term resistance as a descrip- 

 tion of that quality in matter which tends to prevent com- 

 pression ? For if it is a sufficient objection to the term 

 magnetic resistance, it must be a sufficient objection to the 

 term resistance to compression as well, as the analogy be- 

 tween the two cases is so extraordinarily close. It cannot 

 be maintained for a moment that the want of constancy offers 

 any objection. 



An objection may be taken possibly that the magnetic 

 resistance has varying values for rising and falling mag- 

 netizing force, and is therefore not definitely ascertainable as 

 a function of magnetizing force. Answer: We do not give 

 up trying to find out the true values of things because 

 they are superficially complicated with others. If magnetic 



