166 
MINUTES OE PEOCEEDINGS OF 
The electro-ballistic pendulum consists of a short pendulum capable of 
revolving about a horizontal axis through an angle of 75° on each side of 
the vertical through the point of suspension. As a preparation for the 
experiment, the bob is raised to its highest position on one side, where it 
is retained by the attraction of an electro-magnet, whose magnetism is excited 
by a galvanic current so arranged that, when the shot arrives at the first 
screen, it breaks the wire through which the galvanic current was passing, 
and consequently sets the bob free. When the shot reaches a second screen 
it breaks a second galvanic current, and so destroys the power of a second 
electro-magnet which had hitherto supported a small weight. This weight 
now drops, and in its fall completes a third galvanic stream which excites a 
third electro-magnet, and so clamps a light index which had been travelling 
with the pendulum from its position of rest. The index reading is afterwards 
taken, and is supposed to give the angle through which the pendulum had 
moved when the third galvanic stream was closed. An instrument is used 
from time to time to break simultaneously the first and second galvanic 
streams, in order to find the time lost by the falling of the weight, by the 
electro-magnets, &c. One instrument is capable of measuring only one 
short interval of time. The time of describing the various arcs of the 
instrument are calculated from theory, which neglects friction, temperature, 
and the resistance of the air, and depends upon the observed time of a 
mathematically small oscillation of the pendulum. I believe Major Navez 
directs that not less than 1500 vibrations should be counted. Supposing 
there are three to the second, this experiment would last 500 seconds, or 8J 
minutes, an extremely short time in which to determine the rate of an exact 
measurer of time. If the 1500 sufficiently small vibrations were correctly 
counted consecutively , and if the time was noted by a good chronometer, 
there would be an error, greater or less, at the beginning and end of the 
counting. If the vibrations were counted at two or more times, then there 
would be twice as many small errors. The screens are formed of fine copper 
wire through which the galvanic currents are made to circulate. The wire 
is fine in order that it may not obstruct the motion of the ball, and of copper, 
for the sake of its good conducting power. But copper wire is very liable 
to stretch before it breaks. Even the wind or the blast accompanying the 
ball will interfere with the uniformity of its tension. Hence the objections 
that strike one on looking at such an instrument, are:— 
(1) The arrangement of the galvanic streams, varied and complicated. 
(2) The determination of the time table not satisfactory. 
(3) The wires of the screens are almost certain to be unequally stretched. 
(4) The instrument affords no means of checking the accuracy of its 
indications. 
(5) The necessity to employ two or more independent instruments to find 
the resistance of the air. 
Some of these objections might be removed, but the last two appear 
unavoidable. 
