420 Mr. Bosanquet on the Use of the term "Resistance M 



body is said to be less. Consequently resistance diminishes 

 as the change produced increases, and vice versa. 



The change may consist in the flow of a fluid, through a 

 tube say, under pressure; then we should say that the re- 

 sistance offered by the tube to the flow increases as the 

 pressure increases and as the flow diminishes. 



Or, again, we may consider a liquid being evaporated 

 under the action of heat. We may say that the resistance to 

 evaporation is the greater the more heat is required and the 

 less liquid evaporated. 



We may shortly refer to the two elements in any such 

 change as the cause and the effect. And we shall generalize 

 the conception of resistance in stating that it is the greater 

 the greater the measure of the cause and the less the measure 

 of the effect. But nothing is so far assumed as to the precise 

 law which connects these quantities. 



If we now refer to the case of electrical resistance, we 

 have a law suggested by Ohm's law, viz., that the measure 

 of the resistance is the direct ratio of the measures of cause 

 and effect. Or, to take the words " measure of " as under- 

 stood, resistance is the ratio of cause to effect, or the quotient 

 of cause by effect ; or, resistance is proportional to the 

 cause and inversely as the effect. 



This is not the least necessary. We might very well, if 

 there were any reason for it in any particular case, adopt as 

 the measure of resistance such a function as the square, or 

 the square root of the above ratio. It would be quite com- 

 patible with the general definition deduced above from the 

 meaning of the word. But Ohm's law ties us down to the 

 direct ratio. 



If, therefore, we extend the analogy of Ohm's law to other 

 cases, we shall have some such propositions as follows: — 



If force be employed to extend an extensible body or to 

 compress a compressible one, the resistance to extension or 

 compression, on the analogy of Ohm's law, is the quotient of 

 the cause, or force employed, by the effect, or amount of 

 change. 



If liquid flow through a pipe under a difference of pressure 

 between its ends, the resistance, on the analogy of Ohm's 

 law, is the quotient of the cause, or difference of pressure, by 

 the effect, or rate of flow. 



In such cases the precise manner in which cause and effect 

 are measured must be determined by convenience. It does 

 not in the least affect the conception of resistance as the ratio 

 f cause and effect. 



Again, if a liquid be evaporated under the action of heat, 



