On the Graduation of the Sonometer, 



59 



N" 1 . 



U U N-1. 



J, 

 1 



1 



4 



6 



1 

 3 



4 



i 



3 



i 



9 



4 



2 



1 



8 



i 



7 



i 



5 



A 



10 



11 



l 1 ! 



The table of powers of Un N repeats after every ten powers ; 

 values of the form a m must therefore have a unitated to the 

 base 11 and m to the base 10 as a preliminary operation to 

 U n N. The imitation to the base 10 is accomplished simply 

 by striking off the last digit of m, except when it is ; when 

 U 10 m=10. 



By the above means all calculations not involving surd roots 

 can be checked with much greater ease and certainty than by 

 casting out the nines. 



VII. On the Graduation of the Sonometer. By J. H. Poyn- 

 ting, M.A., Fellow of Trinity College, Cambridge*. 



IT seems likely that such valuable results will be obtained 

 by means of Professor Hughes's sonometer, that it is de- 

 sirable that some method should be employed to turn its at 

 present arbitrary readings into absolute measure, so that, for 

 instance, the induced currents caused by different metals in the 

 induction-balance may be measured and compared with each 

 other. 



In Maxwell's i Electricity,' vol. ii. chap, xiv., the general 

 formula is given for the coefficient of induction of one circular 

 circuit on another. Adapting this to the case where two equal 



* Communicated by the Physical Society, having been read on 

 December 13, 1879. 



