324 Journal of Agricultural Research voi. v, no. s 



tube sufficient to ruin the bulb for accurate measurement of temperature. 

 After the bulbs had been in use a short time they would produce an 

 electromotive force as if they were primary or secondary cells. 



In another type of thermometer bulb the resistance wire was inclosed 

 in a thin-walled small-bore copper tube, which was filled with Wood's 

 metal to exclude moisture from the tube and to render the thermometer 

 more sensitive by increasing the conduction of heat to the wire. This 

 bulb did not prove satisfactory because, though the thermal conductance 

 from the water to the wire may have been improved, the sensitiveness of 

 the thermometer was not, the mass of metal apparently causing a lag in 

 response to temperature change. Furthermore, the Wood's metal 

 apparently did not completely exclude moisture, for ultimately the wire 

 in this thermometer also became short-circuited mth the metal. Another 

 serious objection was the possibiUty that the resistance wire might be 

 stretched by the unequal expansion of the metal in which it was embedded. 



The bulb which was finally used mth utmost satisfaction was con- 

 structed in accordance with the specifications of the one developed by 

 Dickinson and Mueller (11, 12, 13) in connection with investigations on 

 calorimetry at the United States Bureau of Standards, which was designed 

 especially for use in determining the temperature at a definite point of 

 liquid flowing in a tube in a continuous-flow calorimeter. The bulbs 

 were designed especially to combine constancy, freedom from lag, and 

 intimate contact with the entire water flow. The platinum resistance 

 wire was wound on a thin strip of mica, and this coil, laid between two 

 similar mica strips, was inclosed in a flat sheath of thin silver which 

 pressed the mica insulating strips firmly against the resistance wire, thus 

 affording opportunity for rapid conduction of heat between the case and 

 the wire. The silver case terminated at the top in a tube which was sealed 

 to a glass tube, on the end of which was a bulb containing phosphorus 

 pentoxid, the purpose of which was to exclude moisture from the space 

 in which the resistance wire was inclosed. The flat part of this bulb, 

 which was about 10 cm. long, 10 mm. wide, and i mm. thick, and con- 

 tained the sensitive part of the thermometer, was inserted in a brass tube 

 with a constricted channel, like that for the final heater described on page 

 319, so that the sensitive portion of the thermometer was surrounded by a 

 space about i mm. across; and water flowing through this space was thus 

 brought into intimate contact with the thermometer, which very rapidly 

 acquired the temperature of the water and responded instantly to changes 

 in temperature and integrated stream lines of temperature, if any existed. 

 The two thermometers, one in the ingoing and the other in the outgoing 

 water, had exactly the same resistance, about 25.5 ohms at 20° C, and 

 the same coefficient of change of resistance with change in temperature, 

 about 0.0039 per degree for the range of temperature in which they would 

 be used, the resistance change of each thermometer being o.i ohm per 



