On the Adjustment of Resistance- Coils. 265 



gation, that the details of the action differ in some respects 

 from those which I have suggested, I think no reasonable 

 doubt can remain that Hall's phenomenon is simply a con- 

 sequence of mechanical action combined with Peltier effects. 

 And if it should be thought that I have, in this paper, 

 brought forward a superfluity of argument in support of a 

 proposition which, when once stated, appears almost self- 

 evident, I can only say that, under the circumstances, and 

 having regard to the great importance of the subject, it 

 seemed desirable that it should receive the most thorough and 

 exhaustive treatment possible. 



XXX. On the Adjustment of Resistance -Coils. By Professor 

 Silvanus P. Thompson, B.A.. D.Sc, Univ. Coll. Bristol*. 



1. A LTHOUGH the existing methods for the exact mea- 

 -£^- surement of the electric resistance of a wire are 

 simple, rapid, and reliable to a very high degree, there is not, 

 so far as I am aware, any simple, rapid, and reliable method 

 yet known of adjusting resistance-coils to their exact value, 

 when some standard coil of known resistance is used for com- 

 parison. It is, of course, much more important to know what 

 the exact value of. a coil is than to have that coil adjusted to 

 one ohm or one hundred ohms, if such adjustment involves 

 some uncertainty in the percentage of its precise value. But 

 I do not see why our standards should not only be accurately 

 known, but also accurately adjusted to standard values, at 

 least with far greater precision than is often the case. Further 

 than this, the great cost of reliable resistance-coils, which hin- 

 ders in many cases their being purchased by individuals and 

 institutions of limited means, arises almost entirely from the 

 high cost of the skilled labour now needed for the tedious 

 processes of adjusting the coils by trial and error to something 

 approaching their nominal values. 



The method of adjusting resistance-coils now suggested has 

 proved itself in practice far more accurate than any other, and 

 the rapidity with which adjustment can be secured is extra- 

 ordinary. It consists in taking as a first approximation a wire 

 cut off roughly of such a length as to have a resistance slightly 

 greater than that to be finally given, and then shunting the 

 whole or a portion of this coil by a coil of much higher resist- 

 ance, also approximately cut off to a resistance calculated from 

 a single careful measurement of the first rough coil. The 

 method, though new for the construction and adjustment of 

 permanent coils, is not new as a method in general, as it was 

 , * Communicated by the Physical Society. Eead February 23, 1884. 

 Phil. Mag. S. 5. Vol. 17. No. 106. April 1884. T 



