414 wenner: wire resistance standards 



These two causes combined therefore account for less than one- 

 half the difference between the radiation intensities measured at 

 Madison and Washington in winter. 



There remains to be considered the relative effect of the dust 

 content of the atmosphere at the two stations, with respect to 

 which no data are available. These pyrheliometric measure- 

 ments lead to the conclusion, however, that on clear days in win- 

 ter, when the northern part of the United States is generally 

 covered with snow, the atmosphere at Madison contains much 

 less dust than the atmosphere of Washington, a difference that 

 does not exist during the summer months. 



ELECTRICITY. — Constancy of wire resistance standards. Frank 

 Wenner, Bureau of Standards. Communicated by E. B. 

 Rosa. 



On account of the convenience and of the high sensitivity 

 readily attainable, electrical methods are being used more and 

 more in various kinds of physical measurements. The numerical 

 values obtained then are in terms of electrical units and the relia- 

 bility of the results depends on the reliability of the electrical 

 standards used. 



In most of the more precise measurements of energy or heat and 

 power as well as in primary electrical measurements with mercury 

 resistance standards and the silver voltameter, and in absolute 

 measurements of resistance and current the results obtained 

 usually involve the resistance of one or more wire resistance stand- 

 ards. In all such and many other measurements the reliability 

 of the results depends upon the constancy, over a longer or shorter 

 time, of the resistance standards used. 



It is therefore highly desirable that our standards should be so 

 constructed as to have as nearly a constant resistance as it is 

 possible to obtain, and that the small changes taking place be 

 known as definitely as it is possible to obtain them. 



At the Bureau of Standards ten 1-ohm resistance standards 

 of the sealed type 1 in which the resistance material is manganin, 

 were selected for use as a reference set. Whenever the resistance 



1 Rosa, Bull. Bureau Standards. 4:413. 1908. 



