56 PROTOPLASM 



(Fig. 43) reveals the construction of the magnet better than 

 words. 



The ingenuity of these micrurgists seems to have no Hmit. 

 Not satisfied with pulling cells apart, injecting salts into them, 

 and determining their electrical potentials, they must measure 

 their temperature. This has been made possible by Whitaker's 

 microthermocouple. Thermocouples of larger size have been 

 known for some time. They are instruments for measuring 



temperature electrically by means of a 

 double electrode of two different metals. 

 A thermocouple consists of a circuit of two 

 different metals joined at two points to 

 form junctions. If the junctions are of 

 different temperatures, an electromotive 

 force exists which is proportional to the 

 temperature difference; it is, however, 

 likewise dependent upon the metals used. 

 Either a galvanometer to measure current 

 or a potentiometer to measure potential 

 thus also measures the temperature 

 difference. The two metals must con- 

 stitute a standardized combination, e.g., 

 iron against platinum, bismuth against an 

 alloy of bismuth and tin, and iron against 

 an alloy of gold and palladium. Such 

 combinations indicate a certain degree of 

 „ .,-.,. ,, temperature when the electromotive force 



Fig. 44. — Microthermo- ^ 



couple of Whitaker, show- recorded is a certain microvoltage (thou- 

 ing successive stages in its gaudths of a volt). These two latter 



construction. ^ 



combinations give respectively 95 to 100 

 and 45 ^lv. per degree of temperature (centigrade). Forty- 

 five microvolts per degree means that 45 jjly. occur in the circuit 

 when the temperature difference of the two junctions is 1°C. 

 Ninety microvolts indicates 2°C., etc. The electromotive force 

 per degree difference is just the same no matter how small the 

 wires or junctions are. The device thus measures temperature 

 difference, but this, of course, becomes temperature direct if the 

 temperature of one junction is set or known; e.g., one junction 

 may be put in a constant-temperature bath or chamber contain- 

 ing a mixture of ice and water at 0°C., whereupon the difference 



