936 J. Trowbridge—Measurement of Strong Electric Currents. 
have the same length. By keeping both forks fixed, and util- 
izing the revolving mirror, which may be regulated by clock- 
work if necessary, any desired constant rate of horizontal mo- 
tion on the screen is secured. Hither or both forks may be 
made to vibrate with constant amplitude by means of a well 
known electro-magnetic attachment. 
Brooklyn, Jan. 31, 1885. 
Art. XXXII.— Measurement of Strong Electrical Currents; by 
JOHN TROWBRIDGE. 
In 1871 I described in this Journal a new form of galva- 
Magazine. It seemed to me then, and I have had no reason to 
change my belief, that a dynamometer is the most suitable 
instrument for measuring strong currents. I have lately, how 
ever, employed the cosine galvanometer for this purpose 10 the 
following manner: The galvanometer is mounted so that Its 
of which is vertical and is in the magnetic meridian or 10 the 
plane of the needle of the compass, whatever that 1s. When 
ment answers as a tangent galvanometer. I then connect the 
t 
4 ? 
resistance so large that the battery resistance can be neglected, 
and having joined the poles in such a manner that the deflec- 
tion produced by the coil of the cosine galvanometer shall , 
opposite to that produced by the current from the dene 
machine in the large outer coil, I incline the coil of the en 
galvanometer until the compass needle is brought again eg 
e then have, if we represent by F and EF” the force a 
duced at the center of the coils by the current from the dynamo 
