20 REPORT— 1868. 
wire surrounding the core of the coil appeared to be quite cool, while the iron core 
itself was considerably heated. The author, therefore, mainly assigned the produc- 
tion of the heat to the cause specified. Now to remove this heat he had perforated 
the two poles of the electromagnet as close as possible to the armatures, and a stream 
of cold water circulates twice round the machine. This carries off the heat 
in a most effective manner, and no appreciable detriment in its electrical results 
occurs. 
The quantity of the mixed gases given off per minute had not yet been ascertained ; 
but it is most interesting to notice the continuance of the decomposition of water 
which takes place for some seconds after all motion in the machine has ceased. 
On the Electric Conductivity of Platinum as affected by the Process of 
Manufacture. By C. W. Stemens, F.R.S. 
On a Permanent Deflection of the Galvanometer-needle by a rapid series of 
equal and opposite Induced Currents. By the Hon. J. W. Srrvrr. 
On the Construction of a Galvanometer for the Detection of weak Electric 
Currents. By F. H. Varzry. 
The great advance which the labours of Sir William Thomson have made in the 
means of determining with precision small equivalents of electrodynamic force is 
sufficiently well known. He has shown the importance and value of using small 
cores upon which the electromagnetic helices are wound, and the advantage of em- 
ploying small magnets for indicating and measuring the amount of force flowing 
through the galvanometer-coil or helix. The small magnet of Sir W. Thomson has a 
mirror attached to it to reflect a beam of light, so that a small motion of the magnet 
gives movement to a line of light thrown upon a darkened screen. It has frequently 
occurred to the author that smaller and lighter magnets could be employed by call- 
ing in the aid of microscopic power. Two instruments were constructed with this 
view. The first consists in suspending by a single filament of silk in the hollow 
core of the galvanometer-coil a magnet of an inverted spur-form, made of the finest 
steel wire that can be obtained, and rendering its motion apparent by viewing it 
through a rectangular prism by means of a microscope, in the eyepiece of which is 
placed a small scale photographed on glass: the magnet appears as a black bar bi- 
secting the field of view; and as the finest wire obtainable for this purpose appears, 
when sufficiently magnified, as thick as a scaffold-pole, it is obvious that the slightest 
motion of the magnet is rendered conspicuous by the image moving to or from over 
the graduated scale in the eyepiece. The second form is more sensitive than the 
first : a small magnet made of flat steel polished on one face is suspended in the 
usual way by a single filament of silk; a small microphotograph of a graduated 
scale is placed at such a distance from the reflecting surface of the magnet-mirror, 
that each division equals two minutes of arc as nearly as possible; the image of the 
scale thus reflected is sent in a line with the optic axis of the microscope; any de- 
flection given to the magnet causes the image of the photographed scale to move 
across the field of view. The reflecting surface moving doubles the apparent motion, 
giving the amount due to the angle of incidence, plus that of reflection. The 
movement of one graduated division being produced by a deflection equal to one 
minute of are, if magnified sixty times by the microscope, wiil render a motion 
equal to one second of are apparent and measurable. When desirable, a small scale 
placed in the eyepiece can be made to give a vernier reading upon the magnified 
scale. The magnifying-power can be increased where desired, and most minute 
amounts of motion rendered measurable. The great difficulty of using instruments 
of such extreme sensibility is due to the interference of extraneous vibration com- 
municated to the small magnets. This, to a great extent, can be overcome by in- 
sulating the various parts from vibration by means of antagonizing springs, and 
preventing the finer vibration from being communicated through the wire itself by 
