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LI. On Bridge Methods for Resistance Measurements of High 

 Precision in Platinum Thermometry. Bij F. E. Smith, 

 A.R.C.Sc* 



(From the National Physical Laboratory.) 

 riTHE object of this communication i s to describe certiin 

 JL new bridge methods of measuring with high precision 

 the resistance of a platinum thermometer, or similar 

 resistance. 



The desirability of investigating the resistance-measuring 

 apparatus of platinum thermometers was first brought to the 

 author's notice by a communication made by Dr. Chree f to 

 the Royal Society. Afterwards Dr. J. A. Barker described 

 sundry troubles met with in precision work, and tests made 

 by ourselves on seN eral resistance bridges for thermometric 

 measurements indicated that many of the difficulties pointed 

 out by Dr. Chree still remained. The present important 

 position of platinum thermometry in stanciardising-labora- 

 tories was a further incentive to this research. 



The accuracy aimed at in high precision platinum 

 thermometry is very considerable. At low temperatures, a 

 measurement within o, 001 C. is not regarded as an 

 extravagant aim, while at higher temperatures, such as 

 800° C, the temperatures are usually recorded within 

 0°-01 0. Between 0° and 100° C. a difference of tempera- 

 ture of 0°*001 corresponds to a change in the resistance of 

 the thermometer of nearly 4 parts in one million, and at 

 800° C. a change of temperature of o, 01 C. corresponds to 

 a resistance change of about 10 parts in a million. 



Dr. Chree in the paper already referred to suggested 

 certain improvements in apparatus to reduce the possible 

 sources of error in platinum thermometry. Dr. Chree used 

 a Callendnr- Griffiths' bridge, the resistance coils of which 

 were of platinum silver, and for accurate work it was 

 necessary to know the temperature of the coils within less 

 than o, l C. He suggested as an alternative to this that the 

 resistances should be made of some material with a small 

 temperature coefficient of resistance. Happily, this is now 

 the case in all modern resistance bridges, manoanin being 

 the material in common use. 



Dr. Chree suggested also that the uncertainties arising 

 out of the plug-contact resistances should, if possible, be 

 reduced, or the resistance of the platinum thermometer 

 increased. The difficulty of securing a sufficiently constant 



* Communicated by Dr. R. T. Glazebrook, C.B., F.R.S. 

 f Chree, Proc. Roy. Soc. vol. lxvii. (1900). 



