II. THERMOMETRY. 229 



For very high temperatures platinum cylinders may be employed instead 

 of copper. 



1074b. Electrical Pyrometer. C. W. Siemens. 



The electrical resistance of metal conductors depends upon their dimensions, 

 material, and temperature ; an increase of the latter causing a corresponding 

 increase of resistance. The law of this increase is known. 



Thus, the resistance of a conductor being ascertained at degree centigrade, 

 it can be calculated for any temperature, and vice versa ; if the resistance can 

 be found by measurement, the temperature can be calculated. This is the 

 principle upon which Siemens' electrical pyrometer is based. 



A platinum coil of a known resistance at degree centigrade is coiled on a 

 cylinder of fire-clay, protected by a platinum shield, which is placed in an iron 

 or platinum tube, and then exposed to the temperature to be determined. 

 Leading wires are arranged to connect this coil with an instrument suitable 

 for measuring its resistance, and from this resistance the temperature can be 

 calculated. 



The instrument supplied for this purpose is a differential voltameter. 



The differential voltameter consists of two separate glass tubes, in each of 

 which a mixture of sulphuric acid and water is decomposed by an electrical 

 current passing between two platinum electrodes. The gas which is 

 generated is collected in the long cylindrical and carefully -calibrated top of 

 the tube, and its quantity is read off by means of a graduated scale fixed 

 behind the tubes. 



Movable reservoirs are provided communicating with the tubes to regulate 

 the level of the liquid. 



The current of the battery is divided (by passing a commutator) into two 

 circuits, one of which consists of standard resistance in the instrument and 

 the platinum electrodes in one tube ; the other, of the resistance to be 

 measured and the electrodes in the other tube. The quantities of gas de- 

 veloped in the two tubes are in inverse proportion to the resistances of their 

 respective circuits, therefore one of the resistances, viz., that in the instrument, 

 being known, the other can be calculated. 



Directions for use : Fill the battery glasses with pure water, or, in case 

 of the power of the battery decreasing, with a solution of sal-ammoniac in 

 water. Connect the poles to B and B' on the commutator. Expose the small 

 end of the pyrometer-tube, as far as the cone, to the heat to be measured, and 

 connect the terminals x, x', c to the ends of the leading cable bearing corres- 

 ponding letters. Connect the other end of the leading cable to the terminals 

 x, x', c on the voltameter. 



The differential voltameter is to be filled with the diluted sulphuric acid 

 through the reservoirs, the india-rubber cushions being lifted from the top of 

 the tubes. The commutator is to be turned so that the contact springs on both 

 sides rest on the ebonite. The liquid in both tubes is to be regulated to the 

 same level (0 degree of scale), and the india-rubber cushions to be let down again 

 Give the commutator a quarter of a turn, and the development of gas will 

 commence almost immediately. Turn the commutator half round every ten 

 seconds to reverse the current. Keep the current passing until the liquid has 

 fallen in the tubes to at least 50 degrees of the scale, then put the commutator 

 n its first position, so that the contact springs rest on the ebonite ; read off 

 the level of the liquids on the scale marked V, and the scale marked V ; find 

 these numbers in the table under V and V, and the intersecting point of the 

 lines starting from these figures gives the temperature. 



For a new experiment adjust the levels as before. 



1075. Apparatus for demonstrating the Expansion of 

 Solids by Heat. 4- Steger, Kiel. 



