650 Proceedings of the Royal Society 
5. That the nitrite must he slowly added to the acidified per- 
manganate. 
6. That with the solutions recommended no fallacy is introduced 
by evolution of oxygen, or by secondary decomposition between 
ammonia or ammonium compounds and nitrous acid. 
2. The Absolute Determination of the Strength of an 
Electric Current by means of the Balance. By Pro- 
fessor James Blyth, M.A. 
The object of this paper is to describe a method of determining 
the strength of an electric current in absolute measure by measuring, 
in grammes weight, the repulsion between two circuits each carrying 
the same current. 
For convenience of calculation, the circuits are made circles of 
equal radius, and are placed with their planes horizontal. 
The construction of the instrument is as follows : — A delicate 
chemical balance is provided, with the scale pans replaced by two 
suspended coils of wire. Each of these coils is made of a single 
turn of moderately fine copper wire bent round a disc of glass of the 
requisite diameter. The ends of the wire, after being firmly tied 
together for a considerable length with a thin layer of insulating 
material between, are bent so as to lie radially along the upper sur- 
face of the disc. This disc is cemented concentrically to a similar 
disc of slightly larger radius, so that the wire is firmly fixed in the 
step formed by the two discs. The double disc has a round hole 
through its centre, by means of which it is attached to the end of a 
wooden cylinder. At opposite sides of this cylinder are fixed two 
vertical rods of brass of equal length, and terminating at the top in 
small platinum cups for holding mercury or dilute acid. To the 
lower ends of these rods, one to each, are soldered the ends of the 
copper wires which pass radially across the disc. The whole is sus- 
pended by a suitable hook from one end of the balance beam, and 
is so adjusted that the cups are in line with the knife edge at the 
end of the beam, and have their upper edges just a little above its 
level. A precisely similar coil is suspended from the other end of 
the beam, and the lengths and weights of both are so adjusted that, 
