210 Mr. F. W. Burstall on the Use of 



present paper deals. There can be no doubt tbat with a 

 bare wire immersed in oil the heating bj the testing current 

 is much less than with covered wire, and further, the tempe- 

 rature of the wire can be accurately measured ; bat perhaps 

 one of the most important advantages in the use of a bare 

 wire lies in the fact that the wire can be annealed in a most 

 perfect manner by the passage of a sufficiently large current 

 to heat it to a red-heat for a few seconds. It has long been 

 known that well-annealed coils are much less liable to change 

 than coils which have not been so treated. Dr. Lindeck, of 

 the Reichsanstalt, has, in his various papers, laid great stress 

 upon the necessity for annealing standard coils, which is 

 effected by heating the coil in an air-bath to about 140°, the 

 highest temperature a silk-covered coil can bear without 

 material injury. The heating of the wire to a red-heat is, of 

 course, much more efficient than the method adopted by 

 Dr. Lindeck for freeing the wire from undue stress. 



The wire which I have employed for the whole of the 

 resistance-box was drawn from one cast of platinum-silver, a 

 material which experience has shown to be the most perfect 

 for resistance-measurement. 



The two forms of resistance-box usually employed are the 

 series form and the dial form. In the former a number of 

 coils are placed in series, and any required number can be 

 short-circuited. This form, though it requires comparatively 

 few coils, has the disadvantage that any fluctuation in one 

 plug causes an error in the final measurement, and if any 

 considerable number be used, it is almost impossible to pre- 

 vent variation in the sum of the contact-resistances. For 

 these reasons it seemed advisable to adopt the second form of 

 box, the dial pattern, in which the number of plugs employed 

 is a minimum, and remains the same whatever resistance is 

 being measured. 



It is not essential to make standard resistances in order to 

 calibrate a resistance-box, as each dial can be measured in 

 terms of the preceding dial. This method, however, perpe- 

 tuates any errors that may have been made in the measure- 

 ment of any of the smaller coils, and it is not possible to 

 measure the coils in the box with the same accuracy that can 

 be attained in the determination of the standard coils. I 

 have therefore constructed standard coils for every dial, 

 and their values have been obtained from a 1 ohm coil inde- 

 pendently of the resistance-box. 



