Note 32: resistances of iron wire and salt water compared 429 



NOTE 32, ARTS. 398, 576, 687. 

 Comparison of the Resistance of Iron Wire and Salt Water. 



Cavendish never published the method by which he made this comparison, 

 but the result given in Art. 398 seems to have been accepted by men of science 

 on Cavendish's bare word, without any question as to how it was obtained. 



It appears fron Art. 576 that Cavendish made his body and the iron wire 

 the branches of a divided circuit, and then tried how many inches of salt water 

 must be put in the place of the iron wire, so that the shock might appear of 

 the same strength. 



By Matthiessen's experiments on the resistance of metals, the resistance of 

 an iron wire of the dimensions given by Cavendish would be about 196 Ohms. 

 As this is much less than that of a man's body from hand to hand, it would 

 have made hardly any difference to the shock whether Cavendish took it through 

 his body alone, or through his body and the iron wire in series. 



By using the iron wire as a shunt and increasing the discharge so as to 

 obtain a shock of easily remembered intensity, Cavendish was enabled to 

 compare the wire with a column 5-1 inches long of saturated solution of salt. 



By this experiment the resistance of saturated solution of salt is 355,400 

 times that of iron. 



By the statements in Art. 398, that the resistance of rain-water is 400,000,000 

 times that of iron wire, and 720 times that of a saturated solution of sea-salt, 

 the resistance of saturated solution would be 555,555 times that of iron wire. 



It is true that this result given by Cavendish does not agree with the only 

 experiment he has recorded, but we must remember that it is the only result 

 which he published, and therefore he must have thought it the best he had. 



By Kohlrausch's experiments on salt solutions combined with Matthiessen's 

 on metals, the resistance of saturated solution of salt is 451,390 times that of 

 annealed iron, when both are at 18 C. The ratio of the resistances would agree 

 with that given by Cavendish at a temperature of about n C. 



The coincidence with the best modern measurements is remarkable. 



NOTE 33, ART. 619. 



Conductivity of Solutions of Salt. 



According to the measurements of Kohlrausch* the electric conductivity k, 

 of saturated solution of sodium chloride, the conductivity of mercury at o C. 

 being taken as unity, is given by the equation 



io 8 k = 1259 ( J + 0-0308^ + O-OOOI46/ 2 ). 

 * Wiedemann's Annalen, Bd. vi. (1879), p. 51. 



