300 The Eev. T. R. Robinson's Experimental Researches on the 



place of observation, a quantity variable in itself, and by no means easy to ascer- 

 tain. Dr. W. Thomson has more recently proposed* one expressed in terms 

 of the mechanical effect to which the current is equivalent ; which, however, 

 must be regarded as a scientific conception, rather than of practical use. A 

 standard, to be available, must be of easy access and application, and in these 

 respects I see no reason for preferring any to one which is in frequent use, the 

 electrolysis of water. The most obvious current unit is that which can decom- 

 pose a grain of water in a unit of time. It seems to me, however, that if a 

 second, or even a minute, be taken as time unit, the values of current will be 

 inconveniently fractional, if an hour, as much too large, and therefore I take 

 five minutes. Adopting this, all that is required to make these rheometers 

 speak a given language is, to note the seconds in which a known volume of the 

 gases is evolved, and reduce it to that due to 300 seconds ; to compute its nor- 

 mal volume G by means of the formulae in treatises of Pneumatics, and measure 

 carefully the deflection <p', then we have 



G X cotan „ , , 



"^^ log-(0-89310) '-^"^"^^^'^'^' 



the experiments for which can be completed in a single day. 



Both ends of the needle are read with direct, and again with reversed cur- 

 rent, to eliminate excentricity and zero errors ; the readings are made with a 

 prismatic microscope, and can be depended on to 2'. 



The rheostat is used in these experiments merely to equalize the current, 

 and therefore has no necessary connexion with their results ; but as in a former 

 communicationf I mentioned its peculiar construction, and promised further 

 details, I take this opportunity of stating my conclusion as to its working. 

 As exhibited to the Academy on that occasion, it consisted of a wire of pla- 

 tinum, whose length was varied by raising it out of mercury, while it was 

 cooled by being surrounded with distilled water ; and I expected that by mea- 

 suring the temperature of this latter fluid I might apply the necessary correction 

 for the change of resistance due to the heat evolved by the passage of the cur- 

 rent. Unless this be attended to, I am satisfied that no measures can be made 



* Philosopbical Magazine, 1851; p. 551. 



f Transactions of the Royal Irish Academy, vol. xxi. p. 303. 



