_Sine and Tangent Galvanometer._--An exhibit of original scientific
apparatus was contributed by Prof. G. F. Fitzgerald, of Trinity College,
Dublin. The first instrument was a sine and tangent galvanometer, which
combines both instruments, and has four interesting peculiarities: (1) The
windings of the coils are visible through the plate glass sides, so as to
be capable of easy measurement _in situ_. (2) The position of the needle
is read by reflections of a cylindrical scale in two rectangular mirrors
whose intersection is horizontal, and which are attached to the magnet.
These mirrors reflect images of opposite sides of the scale to a fixed
mirror which reflects them into a microscope, in which, by means of a
micrometer, it is possible to read accurately the position of the line
which is the same in the two images. (3) This cylindrical scale is affixed
to the base of the instrument, and the coils can be rotated round it, so
that when the instrument is used as a sine galvanometer its position is
read by reflection in the rectangular mirrors attached to the magnet of a
pointer attached to the coils. (4) By a slight modification of the
suspension, a beam of light can be reflected from a mirror connected to
the magnet at 45 deg. to its axis of rotation, and can emerge through the
plate glass side of the instrument and fall on a horizontal scale, where
it will measure the tangent of the deviation instead of the tangent of
twice the deviation, as in ordinary reflecting galvanometers.
The meldometer shown is an instrument for facilitating the identification
of small quantities of minerals by comparative observations on their
melting points, and for observing the phenomena of their fusion and
ebullition. It consists of a strip of platinum arranged to traverse the
stage of a microscope, and heated by a current derived from two Grove's
cells.
On this strip the fragments of the mineral, or, if for comparative
observation, of two or more minerals, are placed. The temperature of the
platinum is then raised by gradually diminishing a resistance placed in
circuit with the battery and meldometer, the behavior of the substance
being meanwhile observed through the microscope. To effect the elevation
of a temperature automatically, a resistance, consisting of a rod of
carbon fitted in a vertical glass tube, is employed. Professor Fitzgerald
showed two sets of apparatus for measuring the densities of gases. Both
methods depend on the determination of the amount by which a body is
buoyed up when immersed in the gas.
_Model for Illustrating the Properties of the Ether._--A very interesting
exhibit was the model for illustrating the electromagnetic and
luminiferous properties of the ether, of which a detailed description is
almost necessary. The model consists of a series of wheels, rotating on
axes fixed perpendicularly in a plane board, and connected together by
India-rubber bands. The axes are fixed at the intersections of two systems
of perpendicular lines, and each wheel is connected with each of its four
neighbors by an India-rubber band. Thus all the wheels can rotate without
any consequent straining of the system if they all rotate at the same
rate. If, however, some of the wheels are rotated through a different
angle from others, the India-rubber bands will be strained. If it be
desired to represent a region in which conducting matter exists, it will
be represented by removing the bands from a set of wheels. Suppose the
bands are removed from the regions, A and B, and from the connecting line,
A B, then we can represent the charging of these regions with opposite
electricities by introducing some mechanism by means of which the wheels
on opposite sides of the line, A B, can be rotated in opposite directions.
The model is not intended to illustrate in any way the connection between
the ether and matter; indeed, one of the advantages claimed for the model
is, that the study of it so distinctly emphasizes the distinction between
the phenomena depending on the general properties of the ether by itself
and those depending on its connection with matter. For instance, from the
very case we have just considered, we get impressed upon us that it is by
means of matter only that we can get a hold on the ether so as to strain
it. As the object is not to illustrate the connection between matter and
ether, any rough method of turning the wheels so as to create the proper
strain will do well enough, as it is not the method of producing, but the
nature of the strain produced that is to be considered. Having once
rotated these wheels, we may replace the bands along the line, A B, and we
have the state of the ether between two oppositely electrified bodies
represented on the model.
It will be observed that half the India-rubber bands are strained, and
that in lines running round the bodies the tight side of a band is always
away from one body and next the other. This represents the polarization of
the ether. The late Prof. Clerk-Maxwell defined polarization as a state in
which the opposite sides of each element are in opposite states. Now, the
opposite sides of each band are in opposite states--one side loose, the
other tight; and so it can very well represent the polarized state of the
ether. The displacement producing the polarization is due to the different
rotation of the wheels carrying the band causing more of the band to be at
one side of the wheels than at the other--less at the tight and more at
the loose side of the pair of wheels, and this represents the electric
displacement producing the polarization. The direction of this
displacement is at right angles to the line of the bands that are
strained, and is out from one body and in toward the other all round.
Considering the other properties of the ether that are represented by the
model, we observe in the first place that during the time polarization is
taking place the wheels are rotating, and that the rate of rotation of the
wheels is proportional to the rate of increase of polarization, and that
the direction of the axis of rotation is perpendicular to the direction of
the displacement. Hence it is seen that the magnetic force is properly
represented by the rate of rotation of the wheels, and its direction by
the axis of rotation. The model, although simple in construction, is very
useful, and its careful study will greatly assist the student in obtaining
definite physical conceptions of many of the more abstruse phenomena
depending on the ether.
_Prismatic Photometers._--Another exhibit was a photometer made of solid
paraffin, or any other translucent substance, invented by Mr. J. Joly, of
the University of Dublin. The arrangement is at once simple and effective.
The instrument depends upon the fact that if a prism be cut from a
translucent body, and so exposed to a source of light that one only of its
faces is illuminated, the light diffused through the substance and
reflected out through the illuminated faces of the prism gives it an
appearance as if lighted up internally. The effect is, in fact, as if the
prism itself was a source of light. Two such prisms laid together on
smooth faces, and receiving light from separate sources, if placed so as
to be at opposite sides of the plane of division, appear as if each was
emitting light proportional in intensity to the source of its supply. The
double prism has the appearance of two luminous bodies laid side by side.
When, however, the supply to each prism is brought to equality, they
appear as if emitting equal quantities of light; and it is hard to detect
any longer that two prisms are being observed, so completely does all
trace of the plane of division disappear. An ingenious piece of apparatus
invented by Mr. Joly was one for carrying out his method of determining
the specific gravity of small quantities of dense or porous bodies. The
method here shown enables the specific gravity to be determined whatever
the density or state of aggregation of the substances, and in extremely
minute quantities, with an accuracy limited only by the sensitiveness of
the chemical balance.
_Telegraphing the Readings of Scientific Instruments._--Another invention
of Mr. Joly was his apparatus for obtaining telegraphically the readings
of meteorological instruments placed at a distance from the observer. This
apparatus may be attached or adapted to the various thermometers, the
barometer, rain gauge, and to other instruments placed in a mountain
station, thus enabling their readings to be taken from a conveniently
placed observatory. Any number of instruments may be worked with perfect
reliability and certainty by the use of three wires only; the only extra
piece of apparatus needed being a disk, carrying insulated contact pieces
arranged round its circumference, to which the wires of the different
instruments are attached. Of these three wires, one serves to put one
after the other of the contacts into circuit with the home station through
the second wire. By this second wire the readings are taken and the
readjustment of the instruments effected. The third wire is for the
indication of the contacts, and is taken from all the instruments to the
galvanometer in the home station.--_Industries._