Trowbridfy and Sheldon — Ni utralization of Induction, 17 



atioDfl oi latitude. We now have also the compensating base 

 apparatus and many perfected forms of the pendulum for the 

 measurement of the force of gravity. In fact in every class of 

 work the errors at present range from one-tenth to one-hun- 

 dredth of what was then considered admissible. 



Add to this that there is no check on the astronomical lati- 

 tudes, which are doubly Important on account of the shortness 

 of the arc ; that the elevation above sea is very uncertain ; that 

 their own observations show an uncertainty of seven or eight 

 feet in the sides of the triangles, and that the arc enters into 

 the determination of the ellipse with great effect owing to its 

 geographical position, and it must be conceded that the geo- 

 detic science of to-day demands the re-measurement of the 

 Peruvian arc. It is high time that the equatorial w T ork be put 

 on the same footing as the other data entering into this im- 

 portant problem. 



C. S. Coast and Geodetic Survey, Washington, D. C. 



Art. II. — Neutralisation of Induction;* by John Trow- 

 bridge and Samuel Sheldon. 



The invention of the telephone drew attention to the extra- 

 ordinary sensitiveness of Faraday's electrotonic state, and 

 immediate attempts were made to construct induction balances, 

 so called, which might serve for quantitative measurements. 

 Thus we have Hughes's induction balance, which had its pro- 

 totype in the balance described in Maxwell's '"Electricity and 

 Magnetism,*' vol. II, §636, due to Felici,f and which differs 

 from Hughes's balance merely in the employment of a galvan- 

 ometer instead of a telephone. By substituting the latter 

 instrument Hughes showed that great sensitiveness could be 

 obtained, and even proposed to adopt an instrument for meas- 

 uring minute amounts of impurities in coins arising from 

 alloys. 



The great difficulty, however, in the employment of Hughes's 

 induction balance in quantitative work arises from the difficulty 

 of getting a good minimum of tone in the telephone. The method 

 that Hughes employed was, briefly, to employ four coils — two in 

 a circuit through which an alternating current or an interrupted 

 current was passed, and two other coils placed contiguous to the 

 coils which were in the interrupted circuit, but in another circuit. 

 By interposing a telephone in the last mentioned circuit, and 



* From the Proceedings of the American Academy of Arts and Sciences, 

 f Nuovo Cimento, vol. ix, p. 345, 1859. 



Am. Jocr. Sci.— Third Series, Vol. XXXIX, No. 229.— Jan., 1890. 

 2 



