BY DR. BURDON-SANDERSON. 347 



rent, and consequent!}- with the difference of temperature be- 

 tween the two junctions, the variations are not proportional, so 

 that, c. p., a deflection of 30 does not indicate a current twice 

 as strong as a deflection of 15. The relation between the read- 

 ings and the intensities of the currents the}' indicate is different 

 in each instrument, and consequently, must be determined once 

 for all for each. Of the various modes which may be adopted 

 for this purpose, the simplest is the empirical method devised 

 originally by Melloni, an account of which will be found in all 

 treatises on physics ; the operation can be best done in a physi- 

 cal laboratory. Within twenty degrees the deflection is usu- 

 ally so nearly proportional to the strength of the current, that 

 the error may be disregarded. For deflections beyond this 

 point the results of the graduation must be recorded in a table 

 of the following form, which must be kept with the instrument. 



Deflection. Intensity of Current. 



20 20.0 



24 25.0 



28 31.5 



32 39.6 



36 49.5 



etc. etc. 



The above numbers are taken from the example given by 

 Melloni. In the second column the starting number 20, stands 

 for the intensity of current indicated by a deflection of 20. 

 This being assumed, the other numbers represent the intensities 

 corresponding to the deflections opposite to which they stand. 



The instrument having been graduated, it is still necessary 

 to determine for each element the constant, by which the start- 

 ing number must be multiplied in order to give the temperature 

 difference. Thus, if with a certain element a deviation of 20 

 is produced by a difference of temperature amounting to 0.10 

 C., the temperature corresponding to any other deviation is 

 obtained by multiplying the number opposite to it in the table 

 by 0.005, which is therefore the constant required. This deter- 

 mination the physiologist must make for himself. It is effected 

 by immersing the junctions into two large vessels containing 

 water or oil, the difference of temperature between which is 

 measured by accurate and sensible thermometers. To avoid 

 error, it is of course necessary to repeat the observation many 

 times. 



[For the accurate measurements of temperature which are 

 required in some physiological and pathological researches the 

 multiplier is not adapted. We substitute for it a true galvano- 

 meter. The instrument used in German}' is the Spiegelbussole 

 of Wiedemann, a description of which will be found in Rosen- 

 thal's " Electricitatslehre fur Mediciner." In England, the 



