232 J. Trowbridge — A Phasemeter. 



divisions in their order, essential characters, and manner of 

 separation, are exactly analogous to those on the south shore of 

 Lake Superior,* and northwest of Lake Superior at Steep 

 Rock Lakef as shown by Smyth. Is it not then highly prob- 

 able that the unconformities at these different districts are the 

 same and that the respective series are or once were continuous 

 for the great triangular area between them, including much of 

 the Lake Superior-Lake Huron region. 



U. S. Geological Survey, 



Madison, Wis., January, 1892. 



Art. XXVIII. — A Phasemeter; by John Trowbridge. 



Certain investigations in magnetism have led me to adopt 

 an instrument which I have termed a Phasemeter and which," 

 it seems to me, is destined to be of use in determining ques- 

 tions of the phase of alternating electric currents in transfor- 

 mers and in branch circuits. This instrument makes use of 

 the method employed by Lissajous to study the vibrations of 

 tuning forks, and afterwards adopted by Helmholtz in his 

 vibroscope, Two telephone diaphragms are provided with 

 mirrors. A beam of light is reflected in such a manner that 

 the vibration of one telephone diaphragm gives a spot of light 

 a horizontal motion, the second telephone gives the spot of 

 light a vertical motion, and the combination of the movements 

 of both diaphragms gives a figure which indicates the relative 

 amplitude of the motions of the diaphragms and also the dif- 

 ference of phase of the currents which set the diaphragms in 

 motion. 



At first I employed a system of levers to amplify the motions- 

 of the diaphragms. The liability of the levers to communicate 

 vibrations peculiar to themselves and thus to superimpose sym- 

 pathetic vibrations on the diaphragms led me to abandon this 

 system, and to affix a stylus to the center of the telephone dia- 

 phragm which touched a little mirror which was delicately 

 placed on a hardened steel point on which it could rock. This 

 method of amplifying the motions of a telephone diaphragm 

 was employed by Professor Eli Blake, of Brown University 

 in 18784 It is a very sensitive arrangement, and I soon 

 perceived that the mirror and its supports needed to be care- 



*This Journal, III, vol. xli, pp. 117 to 137, An Attempt to Harmonize some 

 Apparently Conflicting Views of Lake Superior Stratigraphy, C. R. Van Hise. 



f This Journal, vol. zlii, pp. 317 to 331, Structural Geology of Steep Rock Lake, 

 Ontario, Henry Lloyd Smyth. 



% This Journal, vol. xvi, 1878. 



