140 SCIENTIFIC APPARATUS. 



nishes an example of the inconvenience of the want of uniformity 

 in systems of measurement. 



The thermometers we have hitherto considered depend on the 

 difference of dilatation of a liquid or a gas and the vessel con- 

 taining it. Other thermometers depend on the difference of 

 dilatation of solids. Breguet's thermometer consists of a thin 

 compound strip of metal consisting of three layers, of silver, gold, 

 and platinum respectively. This strip is curled up into a helix, 

 the silver side being outermost. As the temperature rises the 

 silver expands more than the gold, and the gold more than the 

 platinum, and the helix coils itself up. The lower end of the 

 helix carries an index by which its rotation is made manifest. 

 This thermometer, from its exceedingly small mass, acts very 

 promptly, and is well fitted to indicate sudden minute changes of 

 temperature. 



Other thermometric instruments depend on electric phenomena. 



The thermometric current was discovered by Seebeck in 1822. 

 The first thermopile, by which the electromotive force in the 

 circuit was multiplied, was constructed by Orsted and Fourier. 

 It is to Nobili, however, that we owe the development of the 

 power of the thermopile as a method of measuring very small 

 changes of temperature, such as those due to the incidence of heat- 

 radiations. 



Another way in which temperature may be measured by electric 

 effects is by the increase in the electric resistance of metals when 

 their temperature is raised. On this principle is constructed 

 Siemens' Resistance Thermometer, which is especially useful for 

 determining the temperature of places not easily accessible, such 

 as the bottom of the sea. 



A fine platinum wire of considerable resistance is placed in a 

 closed case at the spot whose temperature is to be observed, and 

 is connected by means of thick copper wires with the place 

 where the observer works. A similar wire in a closed case is 

 placed in a vessel of water, and an arrangement is made by which 



