112 
MARVIN. 
Prof. H. L. Callendar,* who has given the instrument its 
present form. 
“ The extensive researches of Dewar and Fleming,! Cailletet and Colar- 
dean,J at very low temperatures, and those of Heycock and Neville $ at 
very high temperatures, taken with those of Callendar and Griffiths, || 
and Harker and Chapuis ^ at ordinary temperatures, establish beyond a 
doubt the remarkable accuracy of this instrument over a wide range of 
temperature.” 
The instrument in its elementary form scarcely requires 
a description. The essential conditions are that the platinum 
wire which serves as the thermometric substance must be as 
pure as possible ; it must be mounted so that in use it is not 
subjected to any strains resulting from expansion or contrac¬ 
tion that can alter its form or resistance, and, obviously, it 
must be protected from any chemical action that can alter 
its electrical qualities. This is especially important in 
metallurgical work at high temperatures, where the influ¬ 
ence of furnace fumes may prove fatal to the constancy of 
the thermometer. The measurement of the electrical resist¬ 
ance is accomplished by the w T ell-known Wheatstone bridge, 
which is given a convenient arrangement for the specific use. 
The change in resistance between the temperatures of melt¬ 
ing ice and the boiling point of water constitutes the funda¬ 
mental interval of the thermometer. The bridge is so arranged 
that this fundamental interval is subdivided conveniently 
into one hundred equal parts or units, each representing a 
degree of temperature, centigrade, on the platinum ther¬ 
mometer. 
The change of resistance does not by any means conform 
to a linear law, and at temperatures other than zero and 
100° C. the platinum scale differs widely from our ordinary 
scales. Fortunately, however, repeated determinations have 
shown that the relation between the platinum scale and the 
*Phil. Trans. Roy. Soc., vol. 177 A, 1887. 
fPhil. Mag., 1893. 
$ Journal de Phys., T. VIII, 1888. 
I Chem. Soc. Trans., 1895. 
|| Phil. Trans. Roy. Soc., vol. 182 A, 1891. 
f Phil. Trans. Roy. Soc., vol. 191 A, 1900. 
