396 Intelligence and Miscellaneous Articles. 
MEASUREMENT OF STRONG ELECTRICAL CURRENTS. 
BY JOHN TROWBRIDGE. 
In 1871 I described in Silliman’s Journal a new form of galvano- 
meter which I called the Cosine Galvanometer. Six or seven years 
after the appearance of my paper, with a large woodcut of my instru- 
ment, the same instrument was re-invented in England by Mr. Obach, 
and the instrument now goes by the name of the Obach galvano- 
meter in England, and is manufactured by the Messrs. Siemens, 
for the measurement of strong electrical currents. 
In the prosecution of an investigation upon dynamo-electric 
machines I abandoned the cosine galvanometer in favour of a 
dynamometer which I invented, and a full description of which 
will be found in the Proceedings of the American Academy of Arts” 
and Sciences, and also in the London Philosophical 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, however, employed the cosine 
galvanometer for this purpose in the following manner :—The 
galvanometer is mounted so that its compass is at: the centre of a 
large circle of wire, the plane of which is vertical and is in the 
magnetic meridian or in the plane of the needle of the compass, 
whatever that is. When the current from a dynamo machine is 
passed through the large vertical coil, which may consist of a 
single wire, the arrangement answers as a tangent galvanometer. 
I then connect the movable coil of the cosine galvanometer with a 
Daniell cell of known electromotive force, place in the same circuit 
a resistance so large that the battery resistance can be neglected, 
and having joined the poles in such a manner that the deflection 
produced by the coil of the cosine galvanometer shall be opposite 
to that produced by the current from the dynamo machine in the 
large outer coil, I incline the coil of the cosine galvanometer until 
the compass-needle is brought again to zero. 
We then have, if we represent by F and EF’ the force produced 
at the centre of the coils by the current from the dynamo machine 
and by the Daniell cell, S and 8! the respective currents, r and 7’ 
the radii, x and n’ the number of coils in the two galvanometers, 
and H the horizontal force of magnetism :— 
2anSH 2nv'S'H S'n' r 
= Per cos a, or Se COS a. 
F= 
The strength of the current from the dynamo is thus simply 
obtained in terms of the current trom the standard Daniel] cell, 
and the method is independent of the strength of the earth’s 
magnetism, or of the special field in which the instruments may be 
placed. The diameter of the outer coil can be diminished by 
employing Professor Brackett’s method of passing the current 
through one coil in one direction and through an inside coil in 
another. In an experimental trial I employed simply a scaffolding 
of wood, upon which a single turn of wire was fixed as a vertical 
circle.—Silliman’s American Journal, March 1885. 
