J. Trowbridge—Gaugain’s Galvanometer. 383 
Blue Ridge, as Willcox has recently done (Proc. Philad. Acad., 
1874, p. 165), for the more thorough metamorphism of the 
eastern range is sufficient to account. for its greater resistance 
to erosion during the earlier ages. ce this subject of erosion, 
I hope to speak ‘further, at another tim 
I must here express a acknowledgments for local information furnished me 
by Prof. E. B, Olmsted, of Murphy, Mr. J. Mack Whittaker, of Valleytown, Dr. 
Josiah Curtis, now of Washington n, D. C., Messrs. J. R. Dean and R. R. Asbury, 
of White County, Ga. M. F. Stephenson, of Gainesville, R. J. Gaines, of Car- 
rollton, Dr. George Litoe, ae Geologist of Georgia, and Mr. W. L. Nic cholson, 
Topographer of the Post Office Department, Washington, D, C. 
Knoxville, Tenn., March 27th, 38% 
Art. XLL—Brief Coniabunens se the Physical Laboratory of 
Harvard College. No. 15.—Un the construction of Gaugain’s 
Galvanometer ; by JOHN ies IDGE. 
In this instrument the law of the proportionality of the 
strengths of the electric currents, peer gti t eae the 
2 2 Ber 
et 32 | 15/?D tae 
Be Ch): spas ae ptt aps 
in which ¢=strength of current. 
T=horizontal intensity of the earth’s magnetism. 
R=radius of coil or circular wire. 
/=length of needle. 
D=distance from center of needle to center of coil. 
/ eg =/?+D?2+R?. 
=deviation of needle. 
We see from eq. (1) that when = —4D?)=0, or D=* , that 
1 i 
the terms in the parenthesis are of the order e )’ and when is 
small these can be neglected. 
In constructing the galvanometer the serious question arises : 
how much will a small error in the measurement of D an 
of R affect the ae: and which will have the greater effect, 
4n error in the measurement of R or in that of D? 
