PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 659 
irregularities having been found to be much diminished, experiments were made in 
the following manner. 
25. Four of the large iron cells, arranged as a single galvanic element, were used 
to excite the current. The experiment lasted about two hours, during which the 
current was sent through the conductor for twenty minutes at a time, alternately in 
the two directions, twice in each direction. Several minutes were spent in changing 
the direction of the current; the stiffness of the electrodes, and the clamps used for 
the connexions which had to be changed, rendering the process very troublesome. 
Readings of the thermometers were taken at intervals of five minutes during the flow 
of the current in both directions, as well as for some time before the current com- 
menced and after it ceased. 
26. The results of this experiment manifested, among great irregularities in the 
indications of the thermometers, a very decided differential variation between the 
two every time the direction of the current was changed ; and appeared so pro- 
mising, that a series of further experiments on the same copper conductor, and on 
the iron conductor similarly arranged, were immediately commenced, for the pur- 
pose of testing decisively the conclusion which had been indicated, and for discover- 
ing the corresponding effect in iron. To avoid the loss of time and the derange- 
ment in the position of the conductor by the shifting of the heavy clamps and stiff 
electrodes between its ends, in changing the direction of the current, a commutator, 
by which the change could be effected nearly instantaneously, was constructed on 
the following plan. 
27 . Four square holes, each of 1 inch, in a square block of mahogany, were fitted 
with bottoms of thick copper slabs, passing through the mahogany, and cemented 
MDCCCLVI. 4 s 
