i68 



Theodolite, Transit, Hoskold's. A special form of theodolite or 

 tacheometer for mining and underground railway work. 

 [13, 25, 27.] 



Theodolite, Transit, Mountain. A theodolite designed for geo- 

 graphical exploration and for making sketch surveys. [2, 

 6, 13, 17, 21, 24, 25, 26, 27.] 



Thermal Conductivity of Copper (Searle's Apparatus). For 

 the absolute measurement of the thermal conductivity of 

 copper. [19.] 



Thermatelephone. A telephone dependent for its action on the 

 difference in temperature of the surface of a diaphragm, 

 when it is convex or concave, as measured by a thermopile. 



Thermetograph. A self-registering thermometer, especially one 

 that registers the maximum and minimum during long 

 periods. See also Thermometer. [13, 17, 20, 21.] 



Thermo-Ammeter, Duddell's. An instrument for the measure- 

 ment of very small currents. The current to be measured 

 passes through a heater, which becomes heated in conse- 

 quence and warms a thermo-couple, causing a current to 

 flow through a coil turning in a magnetic field and carrying 

 a pointer. [4.] 



Thermo-Barometer. An instrument for measuring altitudes by 

 means of determining the boiling point of water. See also 

 Barometer, Hypsometer. [6, 13, 17, 27.] 



Thermo-Couple. Two wires of dissimilar metals joined at one 

 end and forming part of an electric circuit. When the 

 junction is heated, a current flows in the electric circuit ; 

 hence, if this current is measured by a galvanometer, it is 

 possible to employ thermo-couples as a means of measuring 

 temperatures. See Thermo-Electric Pyrometer. [4, 16.] 



Thermo-Couple Potentiometer. A potentiometer (q.v.) for 

 measuring the potential of thermo-couples. [16.] 



Thermo-Electric Pyrometer. See Pyrometer. [4, 16.] 



Thermo-Electrometer. An instrument for ascertaining the 

 heating power of an electric current. [13.] 



Thermo-Galvanometer, Duddell's. An instrument for the 

 measurement of extremely small alternating or continuous 

 electric currents to a high degree of accuracy. It consists 

 of a resistance which is heated by the current to be 

 measured, some of the heat thus generated heating the 

 thermo- junction of a Boys' radio-micrometer (q.v.}. [4.] 



Thermograph. An instrument for automatically recording on 

 a chart variations of temperature. [2, 4, 5, 9, 13, 15, 16, 

 17, 20, 21, 24, 27, 28.] 



Thermo-Hydrometer. A thermometer and hydrometer combined 

 in one instrument. [5, 13, 16, 17, 20, 21.] 



Thermometer. An instrument for the measurement of 

 temperature, commonly founded on the principle that 

 changes in the temperature of a body are accompanied by 



