142 LAWRENCE R. PROUTY AND JAMES D. HARDY 



of this metal are at the same temperature. Measuring devices such 

 as galvanometers or potentiometers may thus be inserted in the cir- 

 cuit without the creation of large extraneous thermoelectric forces. 



For precise work, it is necessary to have as much of the external 

 measuring circuit of the same metal as possible and, for this purpose, 

 special galvanometers with all-copper circuits are available for use 

 with copper-constantan thermocouples. It is also advisable to in- 

 vestigate the parts of the circuit likely to give rise to troublesome 

 thermoelectric voltages. This can be done by closing the circuit and 

 placing the hand near switch contacts, soldered connections, etc. 

 When extraneous sources of e.m.f.'s are discovered in this manner, 

 resoldering of contacts after careful cleaning and scraping will usually 

 eliminate the difficulty. 



For taking temperatures of small masses, thermocouples are very 

 satisfactory. They may be constructed of metal plates or wire of 

 any gage. It is possible to make junctions of extremely fine wire or 

 metallic films, with the aid of a dissecting microscope. These are 

 useful in measuring the temperatures of bacterial colonies, insects, 

 plants, or temperatures within the various organs and blood vessels 

 of animals and man. 



Thermocouple wires should be thoroughly cleaned, and the ends 

 twisted together and soldered with a minimum of solder. No solder- 

 ing flux should be used that might cause the couples to deteriorate or 

 vary in thermoelectric properties with time. After soldering, the 

 wires should again be cleaned with an organic solvent such as alcohol, 

 ether, or acetone. The "reference" junction may then be dipped into 

 enamel, Ambroid, or other material to form a thin waterproof coating. 

 Another method of protecting this junction is to fill a glass capillary 

 with melted wax and insert the junction so that it rests within the thin 

 tip of the capillary. The "reference" junction is then inserted into a 

 vacuum bottle or Dewar flask to the same depth and relative position 

 as the bulb of a calibrated mercury thermometer. The thermocouple 

 and the thermometer are supported by a cork or rubber stopper fitted 

 into the container. In no case should either of these touch the bot- 

 tom or sides of the container. Three additional holes in the stopper 

 will provide for insertion of a funnel, a siphon provided with a pinch- 

 cock or clamp, and a stirring device. The latter may be rotated by 

 manipulation of a string looped about the shaft or by an electric 

 motor. If the stirring is too rapid, mechanical heat may be gener- 

 ated and alter the temperature within the reference flask. 



