240 
PROFESSOR TYNDALL’S FURTHER RESEARCHES 
M. V. Feilitzsch pushes his experiments further, and finds that when the bismuth is 
motionless within its spiral, the position of the magnet is just the same as when the 
bismuth is entirely withdrawn ; hence his final conclusion, that the deflection of the 
magnet in Weber’s experiments is due to induced currents, which are excited in the 
bismuth by its mechanical motion up and down within the spiral. 
These divergent opinions upon a question of such vital beating upon the general 
theory of magnetic phenomena, naturally excited in me the desire to make myself 
acquainted with the exact value of M. Weber’s experiments. The most direct way 
of accomplishing this I considered to be, to operate with an instrument similar to 
that made use of by Weber himself ; I therefore resolved to write to the constructor 
of his apparatus, but previous to doing so the thought occurred to me of writing to 
M. Weber, to inquire whether his further reflections on the subject had suggested 
to him any desirable modification of his first instrument. In reply to my question he 
undertook to devise for me an apparatus, surpassing in delicacy any hitherto made 
use of. The design of M. Weber was ably carried out by M. Leyser of Leipzig, 
and with the instrument thus placed in my possession I have been able, not only to 
verify the experiments of M. Weber, but to satisfy the severest conditions proposed by 
those who saw in the results of these experiments the effects of ordinary induction. 
DESCRIPTION OF APPARATUS. 
A sketch of the instrument made use of in the present investigation is given in fig. 2. 
BO, B'O' is the outline of a rectangular box, the front of which is removed so as to 
show the apparatus within. The back of the box is prolonged, and terminates in two 
semicircular projections, which have apertures at H and H'. Stout bolts of brass, 
which have been made fast in solid masonry, pass through these apertures, and the 
instrument, being secured to the bolts by screws and washers, is supported in a ver- 
tical position, being free from all disturbance save such as affects the foundations of 
the Royal Institution. All the arrangements presented to the eye in fig. 2 are made 
fast to the back of the box, but are unconnected with the front, so as to permit of 
the removal of the latter. WW' are two boxwood wheels with grooved peripheries, 
which permit of motion being transferred from one wheel to the other by means of a 
string ss. Attached to this string are two cylinders, mn, op, of the body to be exa- 
mined : in some cases the cylinders are perforated longitudinally, the string passes 
through the perforation, and the cylinders are supported by knots on the string. HE, 
H'E' are two helices of copper wire overspun with silk, and wound round two brass 
reels, the upper ends of which protrude from H to G, and from H' to G'. The inter- 
nal diameter of each helix is 0‘8 of an inch, and its external diameter about 1*3 inch ; 
the length from H to E is 19 inches, and the centres of the helices are 4 inches apart ; 
the diameters of the wheels WW' being also 4 inches. The cross bar GG' is of brass, 
and through its centre passes the screw R, from which depends a number of silk 
