120 REPORT—1863. 
necessary corrections applied in the calculations. The Huyghens’ gearing for 
the driving hand-winch was somewhat roughly constructed, and could certainly 
be improved; nevertheless there was little difficulty in maintaining a sensibly 
constant driving-power for twenty minutes at a time. The speed of the coil 
was controlled by a frictional governor of novel form, designed by Mr. Jenkin 
for another purpose, and lent for the experiments in question. The action of 
this governor, combined with that of the driving-gear, was such that in many 
experiments the oscillations in deflection due to a change of speed were not 
so great as those due to the passage of steamers in the river when all parts of 
the apparatus were at rest; so that the deflections during twenty minutes 
could be quite as accurately observed as the slightly imperfect zero-point 
from which they were measured. Still better results are expected with a 
larger governor, made specially for the apparatus, on the joint plans of Pro- 
fessor Thomson and Mr. Jenkin. The oscillations produced by the passage 
of steamers on the Thames at no great distance from the place of experiments 
were of very sensible magnitude; and although by carefully observing the 
limit of every oscillation during every experiment the error due to this cause 
was in great part eliminated, it is desirable that any future experiments should 
be conducted in some spot free from all local magnetic disturbance. 
The speed of the coil was determined by observing on a chronometer the 
instant at which a small gong was struck by a detent released once in every 
hundred revolutions. Mr. Balfour Stewart’s skill in this kind of observation 
enabled him thus to determine the velocity with great accuracy, especially 
as the observations frequently lasted for twenty minutes without material 
alteration in the speed. 
Some error was apprehended in the necessary measurement of the length 
of the copper wire used, owing to the extension that would be caused by the 
strain usually required to straighten the wire. This really serious difficulty 
was eluded almost by accident, in a manner amusing from its simplicity. 
At the conclusion of the experiments, the wire to be measured was uncoiled 
in the Museum at King’s College and lay in awkward bends on the planked 
floor. The straight planks formed an obvious contrast to the crooked wire, 
and a joint between the planks was found where the opening was just suffi- 
cient to hold the wire when pushed into this little groove. Held in this way, 
the wire when measured was quite straight, and yet was never stretched. 
No other measurements than those already described are required by the 
simple theory ; but this theory, as hitherto stated, stands in need of various 
slight corrections. The currents induced by the earth’s magnetism are modi- 
fied by the currents induced from the little suspended magnet, and also by 
the induction of the coil on itself. The force deflecting the magnet is also 
modified by the lateral distance of the coils from the vertical axis. An ela- 
borate analysis of the corrections required on these grounds was made by 
Professor Maxwell (Appendix D.) ; and to allow of these corrections, the mo- 
ment of the suspended magnet was measured, and the position of every turn 
of the copper coil carefully observed. An experimental determination of the 
induction of the coil on itself, by a method due to Professor Maxwell, agreed 
with the calculated correction within one quarter per cent. 
The resistance of the copper coil measured by these laborious experiments 
varied each day, and during each day, according to the temperature; and, 
moreover, this temperature could at no time be determined with sufficient ac- 
curacy. It was therefore intended that at each experiment a small German- 
silver coil, at a known temperature, should have been prepared exactly equal 
in resistance to the copper coil during that experiment, and these small coils 
