[ 410 ] 



XL. Notes on the Measurement of some Standard Resistances. 

 By R. T. Glazebeook, M.A., F.R.S., Director of the 

 National Physical Laboratory *. 



THE problem o£ comparing together two nearly equal 

 resistance -coils is a much simpler one than that of 

 determining the resistance of a coil of 10, 100, or 1000 ohms 

 in terms of the unit coils. There are various methods by 

 which such comparisons may be made. I had occasion some 

 little time back to determine the resistance of certain coils of 

 10, 100, and 1000 ohms. I made each determination pur- 

 posely by two or more methods; and it may be useful to put 

 the results on record with a view of indicating the accuracy 

 which can be reached without any very elaborate precautions. 

 The coils in question were all of platinum-silver with a tem- 

 perature-coefficient of about '00028 per 1° C; and by far the 

 most important source of error is the uncertainty of the tem- 

 perature. 



The 10-ohm coils were measured in three different ways. 

 The first (Method I.) was that employed in constructing 

 the standards of the British Association, and is described in 

 the Report of the B. A. Committee for 1883. 



Three 3-ohm coils are wound on the same bobbin. The 

 bobbin is enclosed in a box one face of which is of ebonite 

 through which the six electrodes of stout copper rod project. 

 By means of an arrangement of mercury cups these three 

 coils can be placed in series or in multiple arc. I have 

 acquired the habit of calling this box with its projecting legs 

 the Beetle, and denoting its resistance when in multiple arc 

 by the letter B. 



Let the resistances of the three coils be 3 + a, 3 + /3, and 

 3 + 7 respectively. Then 



1 x +-U+ ' 



B 3 + « 3 + /3 3 + 7 



-l-^(« + /3 + 7 ) + ^ 7 (« 2 + /3 2 + 7 2 ), 



omitting higher powers of the small quantities a, /3, 7. 

 Hence 



B=l+ y( a + / 8 + 7 )-^ 1 ( a 2 + y3H7 2 -«yS-/37- ya ). 



Hence if a, /3, and 7 are nearly equal, the third term is 



* Communicated by the Physical Society : read May 25, 1900. 



