44^ Proceedings of Societies . [June, 
it, could be made use of for determining the velocity of running 
machinery. They made the magnet revolve, and obtained the 
angle of deflection of a disk suspended by a torsion thread (the 
hair-spring of a watch). They found, as Snow Harris and 
others found before, that, other things being equal, the drag is 
directly proportional to the speed, so that if the torsion of the 
thread could be relied on, and the strength of the magnet did 
not change, a perfect velocimeter could be constructed. They 
consider that this method is better than observing the deflection 
of a magnet over a revolving disk, as in this case they are 
limited to less than a right angle, and changes in the absolute 
magnetism of the earth would affect the results. They also de- 
termined the effect of change of distance, thickness, diameter, 
and nature of the disk, &c., their results agreeing with those of 
former experimenters. They observed that the effect of concen- 
tric circular cuts was far greater than that of even many radial 
cuts ; and that when radial sectors were entirely separated from 
each other the effect was much less than when these were united 
at the centre. They then experimented on liquids by suspending 
a sphere or cylinder of the liquid between the poles of a revolving 
electro-magnet, and succeeded in getting a decided and measur- 
able effect. The importance of this is very great, for they have 
thus a means of determining the conductivity of liquid electro- 
lytes by currents induced in the liquid without the use of electrodes 
and without polarisation. 
Dr. Guthrie stated that as the push on the liquid is directly 
proportional to the current quantity, they hope to measure the 
conductivities of liquids, and connect these to the conductivity 
of solids through the intervention of mercury. 
Prof. Sylvanus Thompson then communicated five laboratory 
notes from University College, Bristol. The first related to the 
source of sound in the Bell telephone receiver. Two theories 
are now being discussed as to this effect : the molar theory re- 
gards the motion of the diaphragm in mass as the source of 
sound ; the molecular theory finds it in the molecular motions of 
the magnetic core of the instrument. Prof. Thompson applied 
his method of getting magnetic curves with iron filings dusted 
on gummed glass to this problem. He found that when no cur- 
rents passed in the telephone the magnetic lines springing from 
the pole of the magnet are gathered together on the diaphragm 
opposite, over a central region, which is magnetised lamellarly, 
or like a magnetic shell. The rim of the plate beyond this 
region is, however, magnetised radially, and between these two 
zones there is a neutral circle. It was remarkable, too, that the 
lines of force touching the plate were bent back around the 
circle, forming a kind of valley. When the current passed in 
the coil in a direction so as to reinforce the magnetism, the lines 
are gathered more closely on the central region of the plate. If 
the current diminishes the magnetism the lines are, on the other 
