I have the honor this evening of addressing a few remarks to you upon
the subject of influence machines, and the manner in which I propose
to treat the subject is to state as shortly as possible, first, the
historical portion, and afterward to point out the prominent
characteristics of the later and the more commonly known machines. The
diagrams upon the screen will assist the eye to the general form of
the typical machines, but I fear that want of time will prevent me
from explaining each of them.
In 1762 Wilcke described a simple apparatus which produced electrical
charges by influence, or induction, and following this the great
Italian scientist Alexander Volta in 1775 gave the electrophorus the
form which it retains to the present day. This apparatus may be viewed
as containing the germ of the principle of all influence machines yet
constructed.
Another step in the development was the invention of the doubler by
Bennet in 1786. He constructed metal plates which were thickly
varnished, and were supported by insulating handles, and which were
manipulated so as to increase a small initial charge. It may be better
for me to here explain the process of building up an increased charge
by electrical influence, for the same principle holds in all of the
many forms of influence machines.
This Volta electrophorus, and these three blackboards, will serve for
the purpose. I first excite the electrophorus in the usual manner, and
you see that it then influences a charge in its top plate; the charge
in the resinous compound is known as negative, while the charge
induced in its top plate is known as positive. I now show you by this
electroscope that these charges are unlike in character. Both charges
are, however, small, and Bennet used the following system to increase
them.
Let these three boards represent Bennet's three plates. To plate No. 1
he imparted a positive charge, and with it he induced a negative
charge in plate No. 2. Then with plate No. 2 he induced a positive
charge in plate No. 3. He then placed the plates Nos. 1 and 3
together, by which combination he had two positive charges within
practically the same space, and with these two charges he induced a
double charge in plate No. 2. This process was continued until the
desired degree of increase was obtained. I will not go through the
process of actually building up a charge by such means, for it would
take more time than I can spare.
In 1787 Carvallo discovered the very important fact that metal plates
when insulated always acquire slight charges of electricity; following
up those two important discoveries of Bennet and Carvallo, Nicholson
in 1788 constructed an apparatus having two disks of metal insulated
and fixed in the same plane. Then by means of a spindle and handle, a
third disk, also insulated, was made to revolve near to the two fixed
disks, metallic touches being fixed in suitable positions. With this
apparatus be found that small residual charges might readily be
increased. It is in this simple apparatus that we have the parent of
influence machines (see Fig. 1), and as it is now a hundred years
since Nicholson described this machine in the Phil. Trans., I think it
well worth showing a large sized Nicholson machine at work to-night
(see Fig. 11, above).
[Illustration: Figs. 1-9.]
In 1823 Ronalds described a machine in which the moving disk was
attached to and worked by the pendulum of a clock. It was a
modification of Nicholson's doubler, and he used it to supply
electricity for telegraph working. For some years after these machines
were invented no important advance appears to have been made, and I
think this may be attributed to the great discoveries in galvanic
electricity which were made about the commencement of this century by
Galvani and Volta, followed in 1831 to 1857 by the magnificent
discoveries of Faraday in electro-magnetism, electro-chemistry, and
electro-optics, and no real improvement was made in influence machines
till 1860, in which year Varley patented a form of machine shown in
Fig. 2. It also was designed for telegraph working.
In 1865 the subject was taken up with vigor in Germany by Toepler,
Holtz, and other eminent men. The most prominent of the machines made
by them are figured in the diagrams (Figs. 3 to 6), but time will not
admit of my giving an explanation of the many points of interest in
them; it being my wish to show you at work such of the machines as I
may be able, and to make some observations upon them.
In 1866 Bertsch invented a machine, but not of the multiplying type;
and in 1867 Sir William Thomson invented the form of machine shown in
Fig. 7, which, for the purpose of maintaining a constant potential in
a Leyden jar, is exceedingly useful.
The Carre machine was invented in 1868, and in 1880 the Voss machine
was introduced, since which time the latter has found a place in many
laboratories. It closely resembles the Varley machine in appearance,
and the Toepler machine in construction.
In condensing this part of my subject, I have had to omit many
prominent names and much interesting subject matter, but I must state
that in placing what I have before you, many of my scientific friends
have been ready to help and to contribute, and, as an instance of
this, I may mention that Prof. Sylvanus P. Thompson at once placed all
his literature and even his private notes of reference at my service.
I will now endeavor to point out the more prominent features of the
influence machines which I have present, and, in doing so, I must ask
a moment's leave from the subject of my lecture to show you a small
machine made by that eminent worker Faraday, which, apart from its
value as his handiwork, so closely brings us face to face with the
imperfect apparatus with which he and others of his day made their
valuable researches.
The next machine which I take is a Holtz. It has one plate revolving,
the second plate being fixed. The fixed plate, as you see, is so much
cut away that it is very liable to breakage. Paper inductors are fixed
upon the back of it, while opposite the inductors, and in front of the
revolving plate, are combs. To work the machine (1) a specially dry
atmosphere is required; (2) an initial charge is necessary; (3) when
at work the amount of electricity passing through the terminals is
great; (4) the direction of the current is apt to reverse; (5) when
the terminals are opened beyond the sparking distance, the excitement
rapidly dies away; (6) it does not part with free electricity from
either of the terminals singly.
It has no metal on the revolving plates, nor any metal contacts; the
electricity is collected by combs which take the place of brushes, and
it is the break in the connection of this circuit which supplies a
current for external use. On this point I cannot do better than quote
an extract from page 339 of Sir William Thomson's "Papers on
Electrostatics and Magnetism," which runs: "Holtz's now celebrated
electric machine, which is closely analogous in principle to Varley's
of 1860, is, I believe, a descendant of Nicholson's. Its great power
depends upon the abolition by Holtz of metallic carriers and metallic
make-and-break-contacts. It differs from Varley's and mine by leaving
the inductors to themselves, and using the current in the connecting
arc."
In respect to the second form of Holtz machine (Fig. 4) I have very
little information, for since it was brought to my notice nearly six
years ago I have not been able to find either one of the machines or
any person who had seen one. As will be seen by the diagram, it has
two disks revolving in opposite directions, it has no metal sectors
and no metal contacts. The "connecting arc circuit" is used for the
terminal circuit. Altogether I can very well understand and fully
appreciate the statement made by Professor Holtz in _Uppenborn's
Journal_ of May, 1881, wherein he writes that "for the purpose of
demonstration I would rather be without such machines."
The first type of Holtz machine has now in many instances been made up
in multiple form, within suitably constructed glass cases, but when so
made up, great difficulty has been found in keeping each of the many
plates to a like excitement. When differently excited, the one set of
plates furnished positive electricity to the comb, while the next set
of plates gave negative electricity; as a consequence, no electricity
passed the terminal.
To overcome this objection, to dispense with the dangerously cut
plates, and also to better neutralize the revolving plate, throughout
its whole diameter, I made a large machine having twelve disks 2 ft. 7
in. in diameter, and in it I inserted plain rectangular slips of glass
between the disks, which might readily be removed; these slips carried
the paper inductors. To keep all the paper inductors on one side of
the machine to a like excitement, I connected them together by a metal
wire. The machine so made worked splendidly, and your late president,
Mr. Spottiswoode, sent on two occasions to take note of my successful
modifications. The machine is now ten years old, but still works
perfectly. I will show you a smaller sized one at work.
The next machine for observations is the Carre (Fig. 8). It consists
essentially or a disk of glass which is free to revolve without touch
or friction. At one end of a diameter it moves near to the excited
plate of a frictional machine, while at the opposite end of the
diameter is a strip of insulting material, opposite which, and also
opposite the excited amalgam plate, are combs for conducting the
induced charges, and to which the terminals are metallically
connected; the machine works well in ordinary atmosphere, and
certainly is in many ways to be preferred to the simple frictional
machine. In my experiments with it I found that the quantity of
electricity might be more than doubled by adding a segment of glass
between the amalgam cushions and the revolving plate. The current in
this type of machine is constant.
The Voss machine has one fixed plate and one revolving plate. Upon the
fixed plate are two inductors, while on the revolving plate are six
circular carriers. Two brushes receive the first portions of the
induced charges from the carriers, which portions are conveyed to the
inductors. The combs collect the remaining portion of the induced
charge for use as an outer circuit, while the metal rod with its two
brushes neutralizes the plate surface in a line of its diagonal
diameter. When at work it supplies a considerable amount of
electricity. It is self-exciting in ordinary dry atmosphere. It freely
parts with its electricity from either terminal, but when so used the
current frequently changes its direction, hence there is no certainty
that a full charge has been obtained, nor whether the charge is of
positive or negative electricity.
I next come to the type of machine with which I am more closely
associated, and I may preface my remarks by adding that the invention
sprang solely from my experience gained by constantly using and
experimenting with the many electrical machines which I possessed. It
was from these I formed a working hypothesis which led me to make my
first small machine. It excited itself when new with the first
revolution. It so fully satisfied me with its performance that I had
four others made, the first of which I presented to this Institution.
Its construction is of a simple character. The two disks of glass
revolve near to each other and in opposite directions. Each disk
carries metallic sectors; each disk has its two brushes supported by
metal rods, the rods to the two plates forming an angle of 90 deg.
with each other. The external circuit is independent of the brushes,
and is formed by the combs and terminals.
[Illustration: Fig. 10.]
The machine is self-exciting under all conditions of atmosphere, owing
probably to each plate being influenced by and influencing in turn its
neighbor, hence there is the minimum surface for leakage. When
excited, the direction of the current never changes; this circumstance
is due, probably, to the circuit of the metallic sectors and the make
and break contacts always being closed, while the combs and the
external circuit are supplemental, and for external use only. The
quantity of electricity is very large and the potential high. When
suitably arranged, the length of spark produced is equal to nearly the
radius of the disk. I have made them from 2 in. to 7 ft. in diameter,
with equally satisfactory results. The diagram, Fig. 9, shows the
distribution of the electricity upon the plate surfaces when the
machine is fully excited. The inner circle of signs corresponds with
the electricity upon the front surface of the disk. The two circles of
signs between the two black rings refer to the electricity between the
disks, while the outer circle of signs corresponds with the
electricity upon the outer surface of the back disk. The diagram is
the result of experiments which I cannot very well repeat here this
evening, but in support of the distribution shown on the diagram, I
will show you two disks at work made of a flexible material, which
when driven in one direction close together at the top and the bottom,
while in the horizontal diameter they are repelled. When driven in the
reverse direction, the opposite action takes place.
I have also experimented with the cylindrical form of the machine (see
Fig. 10). The first of these I made in 1882, and it is before you. The
cylinder gives inferior results to the simple disks, and is more
complicated to adjust. You notice I neither use nor recommend
vulcanite, and it is perhaps well to caution my hearers against the
use of that material for the purpose, for it warps with age, and when
left in the daylight it changes and becomes useless.
[Illustration: Figs. 11 & 12.]
I have now only to speak of the larger machines. They are in all
respects made up with the same plates, sectors, and brushes as were
used by me in the first experimental machines, but for convenience
sake they are fitted in numbers within a glass case. One machine has
eight plates of 2 ft. 4 in. diameter; it has been in the possession of
the Institution for about three years. A second, which has been made
for this lecture, has twelve disks, each 2 ft. 6 in. in diameter. The
length of spark from it is 13-5/8 in. (see Fig. 12). During the
construction of the machine every care was taken to avoid electrical
excitement in any of its parts, and after its completion several
friends were present to witness the fitting of the brushes and the
first start. When all was ready the terminals were connected to an
electroscope, and the handle was moved so slowly that it occupied
thirty seconds in moving one-half revolution, and at that point
violent excitement appeared.
The machine has now been standing with its handle secured for about
eight hours. No excitement is apparent, but still it may not be
absolutely inert. Of this each one present must judge, but I will
connect it with this electroscope (Figs. 13 and 14), and then move the
handle slowly, so that you may see when the excitement commences and
judge of its absolutely reliable behavior as an instrument for public
demonstration. I may say that I have never, under any condition, found
this type of machine to fail in its performance.
[Illustration: Fig. 13.]
[Illustration: Fig. 14.]
I now propose to show you the beautiful appearances of the discharge,
and then, in order that you may judge of the relative capabilities of
each of these three machines, we will work them all at the same time.
The large frictional machine which is in use for this comparison is so
well known by you that a better standard could not be desired.
In conclusion, I may be permitted to say that it is fortunate I had
not read the opinions of Sir William Thomson and Professor Holtz, as
quoted in the earlier part of my lecture, previous to my own practical
experiments. For had I read such opinions from such authorities, I
should probably have accepted them without putting them to practical
test. As the matter stands, I have done those things which they said I
ought not to have done, and I have left undone those which they said I
ought to have done, and by so doing I think you must freely admit that
I have produced an electric generating machine of great power, and
have placed in the hands of the physicist, for the purposes of public
demonstration or original research, an instrument more reliable than
anything hitherto produced.
* * * * *
VIOLET COPYING INK.--Dissolve 40 parts of extract of logwood, 5 of
oxalic acid and 30 parts of sulphate of aluminium, without heat, in
800 parts of distilled water and 10 parts of glycerine; let stand
twenty-four hours, then add a solution of 5 parts of bichromate of
potassium in 100 parts of distilled water, and again set aside for
twenty-four hours. Now raise the mixture once to boiling in a bright
copper boiler, mix with it, while hot, 50 parts of wood vinegar, and
when cold put into bottles. After a fortnight decant it from the
sediment. In thin layers this ink is reddish violet; it writes dark
violet and furnishes bluish violet copies.
* * * * *