29 J. G. MACGREGOR ON THE MEASUREMENT OF THE 
passed through the arms of the bridge, they were rendered constant in direction in the 
galvanometer branch. 
It is well known that in such an induction machine as the above, the commutator 
must be placed on the axis of the rotating magnet as though the current required a certain 
length of time to reach it. The position it should occupy depends upon the rapidity of 
rotation of the inducing magnet and the resistance of the conductors between the coil 
and the commutator. Accordingly I chose a certain rate of rotation for the magnet and 
kept to it, guiding the rotation by clockwork. It was impossible, however, to render the 
resistance of the circuit constant, as it was to contain the electrolytes under investigation. 
Every measurement, therefore, required a series of preliminary experiments to determine 
the proper position of the commutator for the unknown resistance in circuit. These pre- 
liminary experiments involved an intolerable waste of time. The cause of the apparent 
delay of the current is the change produced in the magnetization of the magnet by the 
induced current.* I was, therefore, compelled to adopt some mode of producing alter- 
nating currents which did not involve induction. I chose the Daniell’s cell and used two 
commutators made to revolve on the same axis, the one to break up the continuous 
current of the Daniell’s cell into alternating currents, the other to bring these currents 
into the same direction in the galvanometer branch. The commutators were carefully 
made for me by M. E. Stohrer, of Leipzig. Their structure will be apparent from the 

diagram. They were a modified form of Poggendorff’s Znversor.f They were similarly 
placed on the axis +, 2, and the wires from the cell D, the galyanometer G, and the 
angles of the bridge were so joined up that the one commutator, C,, was between the Daniell’s 
cell and the angles a, c, and the other, C,, between the angles b, d, and the galvanometer. 
C,;, therefore, broke up into alternating currents the continuous current given by the cell. 
These alternating currents traversed the arms of the bridge; but in the galvanometer 
branch, b d, they were reunited to form a continuous current capable of indicating its 
presence by deflecting the needle. 


* Koosen, Pogg. Ann., LXX XVII, (1852), p. 386. Lenz, Pogg. Ann., XCIL, (1854), p. 128. 
+ Poggendorf, Pogg. Ann., XLV, (1838), p. 385. 
