[ 38 ] 



IV. Note on the Electrical Resistance and the Coefficient of Ex- 

 pansion of Incandescent Platinum. By E. L. NlCHOLS, 

 Ph.D.(Gdttingen)*. 



I. T N the measurement of temperatures above the red heat, 

 J- the platinum pyrometer, in one form or another, is as 

 important as the mercury thermometer at ordinary tempera- 

 tures. The researches already completed, on the electric 

 resistance and the coefficient of expansion of platinum, and 

 on the specific heat of that metal, only serve, however, to 

 remind us of the much that remains to be done before we may 

 hope to attain to even a fair degree of accuracy in the mea- 

 surement of temperatures above 500°. 



The present writer, in order to compare the existing for- 

 mulae for the temperature of platinum from its electric resist- 

 ance with those by means of which the temperature is calcu- 

 lated from the coefficient of expansion, and thus to gain a 

 clearer idea of the relative usefulness of the two methods, has 

 determined the resistance and the corresponding length of a 

 platinum wire at various temperatures between 0° and the 

 melting-point of that metal. 



II. Upon a platinum wire 0*4 millim. in diameter and 100 

 millim. long, at points 55 millim. apart and equally distant 

 from the middle of the wire, two very fine platinum wires 

 were welded. They served to mark the ends of the portion of 

 the wire to be measured, and to make electrical connexion 

 with a shunt containing a sensitive galvanometer. The wire 

 was heated by the current from a battery of forty Bunsen's 

 cells. Its resistance was determined by the following method. 



Fig. 1. 



The wire (AB, fig. 1), 

 together with a tangent- 

 galvanometer (G) and a 

 resistance-box (W), was in 

 in direct circuit with the 

 Bunsen's battery. A very 

 small portion of the cur- 

 rent Avas shunted around 

 ab, the portion of the wire 

 to be tested, and carried 

 through a sensitive sine- 

 galvanometer (</) and 

 through a resistance-coil (iv) of 5000 ohms. 



* From Silliman's American Journal for November 1881, having been 

 read at the Cincinnati Meeting of the American Association for the Ad- 

 vancement of Science, August 1861. 



