sanford: magnetic permeability 63 



voltage, current, watts, and candlepower corresponding to an 

 efficiency of 1.5 wpc are computed by means of the above char- 

 acteristic equations. 



Another use of these equations is in connection with tungsten 

 standards the values of which are known at some particular 

 efficiency (or color). In standardizing other lamps these stand- 

 ards are adjusted to a color match with the lamps to be measured 

 and the values of candlepower, current, etc., for each standard, 

 at the voltage corresponding to that particular color, are deter- 

 mined by means of the characteristic equations. 



Although these equations are simple and easy to handle, their 

 use involves comparatively long and tedious computations. To 

 avoid the necessity of making such computations, there has been 

 computed a set of tables from which, with the aid of an ordinary 

 slide rule, correction factors to reduce values of candlepower, 

 current, etc., from one efficiency to another can be read directly. 



By the method described above, the photometric difficulties 

 due to color difference are dealt with, once for all, in determin- 

 ing the characteristics of the tungsten lamp, and thereafter all 

 measurements are reduced to the photometry of lights of the same 

 color. 



PHYSICS. — The temperature coefficient of magnetic permeability 

 within the working range.^ Raymond L. Sanford. Com- 

 municated by the Bureau of Standards. 



Magnetic measurements at different temperatures within 

 the atmospheric range were made on wrought iron, cast iron, 

 and low carbon steel with different heat treatments. The 

 results of these measurements seem to warrant the following 

 conclusions: 



I. The temperature coefficient of magnetic permeability 

 though small can not be neglected in magnetic measurements 

 of high accuracy. 



II. On account of the wide variation in temperature coeffi- 

 cient, not only for different materials but also for the same 



1 To appear in full in the Bulletin of the Bureau of Standards. ;'-'^#-''- 



^/^^^^o 





