504 Chronicles of Science. | July, 
increase at the lower temperature, and that the diminution of the mag- 
netic foree from 0° to 100° is proportional to its magnetism at 0°. 
With regard to temporary magnetism induced in soft iron, cast-iron, 
and steel bars by means of an electro-magnetic coil, it was found that 
at a bright red heat none of the bars were magnetic. Approximate 
determinations of the descending temperature at which magnetism 
begins to be manifested gave 1,000°. With the steel bar the increase 
of magnetic power takes place at first very rapidly, then for a certain 
time it goes on slowly, and then again follows a period of rapid 
augmentation. With the cast-iron bars the second period of rapid 
increase is also observed, but in a less marked degree ; but with the 
wrought-iron it does not exist. The author believes he may con- 
clude from his experiments that the magnetic properties of iron are 
developed suddenly at a determinate temperature. 
Execrriciry.—A very valuable instrument for the production of a 
constant stream of electricity has been for some months past exhibited 
in the scientific circles of London. It is an electro-magnetic induc- 
tion machine, but unlike ordinary machines of this kind the stream is 
constant in one direction, and it can be produced of any tension or 
quantity that may be required. Many attempts have been made to use 
induced electricity for telegraphy, but they have generally failed be- 
cause the tension of the current is too great, and the electricity is in 
impulses. What has long been wanted is as near an approach to a 
battery current as possible, and of any required tension or quantity 
without multiplying the number of battery cells used. The machine 
must also be perfectly self-acting. The way in which these desiderata 
are effected in the machine now alluded to is by using two series of 
induction coils, which are so arranged that one is being magnetized 
nearly at the same time that the magnetism is subsiding in the other, 
so that the two induced impulses may be said to overlap each other ; 
and though these are in opposite directions, the spools are so arranged 
that in the general induction circuit they flow in the same direction, 
thus making a compound impulse of longer duration, composed of the 
two opposite inductions. Such a compound impulse is produced from 
each induction coil, and by an ingenious arrangement of the commu- 
tator, they are all turned into one direction, producing a slightly un- 
dulating but continuous flow. The machine is made for quantity, the 
inner coils being of number 12 wire, and the outer of number 18. To 
an electrician the very name induction coil speaks danger, as it conjures 
up visions of powerful sparks, many inches long, darting from pole to 
pole, and capable of piercing through considerable thicknesses of 
gutta-percha, or even glass. Experiments have, however, shown that 
such fears are groundless with an instrument of this construction ; 
the two wires may be brought so close together that a considerable 
magnifying power is required to show that there is any space at all 
between them, before a spark will bridge across the interval, and it is 
then of the feeblest and most innocent description, being unaccompanied 
with noise and scarcely visible in daylight. The striking distance is 
less than the thousandth of an inch. 
