COMPARISON OF PLATINUM AND GAS THERMOMETERS. 55 



the terminals P,P, can be quickly adjusted to any value between zero and 100 ohms. 

 In the opposite arm of the bridge, between the terminals C,C 2 , are inserted a fixed 

 resistance of O'l ohm and the calibrating apparatus. This consists of two massive 

 copper blocks of rectangular form, mounted on an insulating lise, each pierced 

 by two holes about a centimetre in diameter, which are well amalgamated and 

 partly tilled with mercury. Into one pair of holes are inserted two round copj>er 

 pillars, across which is soldered a piece of thin manganine strip, and into the other 

 pair <>f holes the lower ends of a thick U-shuped copper rod. A sketch of the 

 calibrator is given in fig. 10. 



A number of strips of different resistances, each mounted between copj>er pillars as 

 sho\\n, are first prepared, the values chosen being equivalent to 2, 4, 8, 12, 16, 20, 

 and 24 centring, movement of the slider. The calibration is commenced by placing 

 the contact-maker to the division 24 to the left, and one of the strips in position on 

 the calibrator. The resistance in the opposite arm of the bridge is then adjusted so 

 that no galvanometer deflection is obtained, and the exact position of the slider 

 noted. The manganine strip remaining untouched, the copper short-circuiting piece 

 is now placed across between the two remaining mercury cups, and the slider is then 

 moved to the right till the balance is again restored. In order to eliminate the effects 

 of any gradual temperature changes, the process is repeated, the readings being made 

 in the reverse order, a similar pair of readings l)eing made for each successive position 

 along the bridge-wire. The results of several series of ol>servations made on different 

 days with each strip are then combined, and the whole set treated precisely as an 

 ordinary calibration of a length or volume, and the curve of corrections prepared. 



It was interesting to compare the results of the complete calibration with those 

 deduced from the observations with the two-centim. column alone, and from two test 

 calibrations made by means of the coils M and N. The general agreement of the 

 ililVerent results was found to l)e satisfactory. 



XVIII. The Resistance Coils. 



The manganine coils were annealed, in accordance with the recommendations of the 

 German Reichsanstalt, by heating them to about 140 for some time and allowing 

 them to cool slowly. This was done in a closed electrically heated space in which 

 the temperature could l>e regulated at will, and the cooling could lie made as slow as 

 desired. 



The ends of each coil were hard-soldered to copper tags of rectangular form 

 previous to the final annealing, these tags being afterwards firmly fastened by 

 i-dinary solder to the stout tinned copper leads connected to the contact-blocks. 



The coils were wound on glass tubes 3 centims. in diameter, which were fastt-n. <! 

 l.y metal strips to wooden cross-pieces supported from the iron framework of tin- 

 resist anc, -box. These tubes were coated with three thin layers of shellac varnish 



