386 Intelligence and Miscellaneous Articles. 



the mercury which tabes its place — less the increase in the resist- 

 ance of the wire and mercury due to increase of temperature. 



Let Z= length of platinum wire exposed at C . 



s=resistance per unit length ( = length of 1°) of wire. 



g= resistance per unit length ( = length of 1°) of mercury. 



Jc= temperature coefficient of platinum. 



h = temperature coefficient of mercury. 

 E = total resistance at 0°. 

 R t = resistance (including all) at t°. 



Then 



R t =R -{s(l-M)-g}t-(Jcs-gh)f. ... (1) 



This equation is not quite rigorous, but the approximation is very 

 close. It is of the form 



E t =E -B<-C*» (2) 



The simplest and best way of dealing with it is to determine the 

 constants of the equation (2) by a series of observations making 

 use of the method of least squares. Prom the result the resistance 

 for any degree of temperature may be calculated, or, better, a curve 

 can be constructed from which the temperature corresponding to 

 any resistance can easily be read. 



The advantage of this method over the use of a simple resistance 

 coil is that the change in resistance accompanying a given change 

 in temperature is much greater, and in fact it may be made as 

 great as one desires. As a result the telephone may be substituted 

 for the galvanometer in the resistance measurements, thus greatly 

 simplifying the apparatus as well as increasing the rapidity with 

 which observations may be made ; or, if preferred, a rnuch less 

 sensitive galvanometer may be used. It also possesses the very 

 great advantage of allowing an increase of delicacy as the range of 

 temperature decreases. For earth temperatures this is very de- 

 sirable, and it will easily be seen that thermometers of this kind 

 can be constructed for a few degrees of range with which, by com- 

 paratively rude processes, the measurements may be correct within 

 a very small fraction of a degree. Tor use in earth temperature 

 measurements the thermometer will be enclosed in a strong brass 

 tube for protection, and the connection with the point of observa- 

 tion made by means of a cable of heavy copper wire. The cable 

 will of course form a part of E in equation (2) ; but as it is a con- 

 stant, the substitution of one cable for another, if necessary, will 

 affect the position and not the form of the calibration curve. Its 

 resistance must be small, relatively, and the influence of tempera- 

 ture upon it may be neglected. 



With this device a temperature observation may be taken in less 

 than a minute, no time being consumed in the preparation of liquids 

 of known temperature at the observing station, as in the use of the 

 thermo- junction or the resistance coil. — Silliman's American 

 Journal, August 1885. 



