A PLATINUM SURFACE AT HIGH TEMPERATURES. 



511 



cautions are not sufficient. To avoid thermo-electric disturbances it is necessary to 

 keep all the working parts of the bolometer at the same temperature. To attain this 

 object the entire instrument was made of metal. The insulation of the films at their 

 upper and lower extremities where they are secured to the carriers is provided for by 

 very thin strips of mica. The working parts of the bolometer were protected by two 

 massive metal covers, D, and D 2 , fig. 8, fitting one over the other, and between which 

 room is left for a water circulation. 



The variations due to convection currents formed one of the most serious 

 difficulties. Following the suggestion of Professor DEWAR, the instrument was 

 constructed to allow of the films being used in a vacuum. As shown in fig. 8, the 

 double cover is bolted down on to the bedplate, E. The joint, is kept covered with 

 oil which is poured into the rim, R. 



Pig. 9. 



BaCtery 



A vacuum can only be used for experiments in which the absorption of the quartz 

 plate, Q, is not a serious objection. The screens, B! and S 2 , are intended to regulate 

 the path of the convection currents when the instrument is used under atmospheric 

 pressure ; they also screen the edges of the film from direct radiation. The two 

 other arms of the bridge are made of manganin wire, the coils being placed in an 

 earthenware vessel filled with oil. 



The resistance of each of these two coils is 20 ohms, and the resistance of each of 

 the bolometer films 23 ohms. The connections of the instrument are diagram- 

 matically shown in fig. 9. The bar, A, is of German silver, and serves to adjust the 

 zero of the galvanometer. The bar, B, between the potential contacts, c and d, is 

 divided into 1000 parts of equal resistance, the length between the potential contacts 

 is 80 centims., the resistance '007730. This bar serves to calibrate the galvano- 



