424 Note 31: measures of sensation 



the quantity of electricity communicated to 154 jars gave a shock of about 

 the same strength, though as Cavendish remarks, "as there is a good deal of 

 difference between the sensations of the two, it is not easy comparing them." 



Here 154 is the 2g power of 6, so that the shock seems to depend rather 

 more on the quantity of electricity than on the degree of electrification. This 

 is the only experiment which Cavendish has worked out to a numerical result. 



By the other experiments recorded in Art. 610, 34^ communicated to 7 rows, 

 gives a shock equal to 22 communicated to one row. This would make the 

 number of jars as the 4-3 power of the charges. By Art. 613 the number of jars 

 would be as the 3-3 power of the charge. 



Cavendish had not the means of producing a steady current of electricity, 

 such as we now obtain by means of a Voltaic battery, so that he could not 

 discover the most important of the facts now known about the physiological 

 action of the current, namely, that the effects of the current, whether in pro- 

 ducing sensations, or in causing the contraction of muscles, depend far more 

 on the rapidity of the changes in the strength of the current, than on its absolute 

 strength. It is true that a steady current, if of sufficient strength, produces 

 effects of both kinds, but a current so weak that its effect, when steady, is 

 imperceptible, produces strong effects, both of sensation and contraction, at 

 the moments when the circuit is closed and broken. 



But although this may be considered as established, I am not aware of any 

 researches having been made, from the results of which it would be possible 

 to determine, from the knowledge of the physical character of two electric 

 discharges, which would produce the greater physiological effect. 



The kind of discharges most convenient for experiments of this kind is that 

 in which the current is a simple exponential function of the time, and of the form 



_* 

 X" Ce"'' , 



where x is the strength of'the current at the time /, C its strength at the beginning 

 of the discharge, and r a small time, which we may call the time-modulus. 



In this case the whole physical nature of the discharge is determined by 

 the values of the two constants C and r. The intensity of the sensation pro- 

 duced by the discharge through our nerves is, therefore, some function of these 

 two constants, and if we had any method of ascertaining the numerical ratio 

 of the intensities of two sensations, we might determine the form of this function 

 by experiments. We can hardly, however, expect much accuracy in the com- 

 parison of sensations, except in the case in which the two sensations are of the 

 same kind, and we have to judge which is the more intense. 



According to Johannes Miiller, the sensation arising from a single nerve 

 can vary only in one way, so that, of two sensations arising from the same nerve, 

 if one remains constant, while the other is made to increase from a decidedly 

 less to a decidedly greater value, it must, at some intermediate value, be equal 

 in all respects to the first. 





