![[IMAGE]](contacts/fig56.gif)
162. The Gravity Battery.--- Several modifications of the Daniell battery (19), especially adapted to telegraphic use, are finding much favor within the past few years. The most economical and generally useful of these improved forms is the gravity battery. The best arrangement is that known as the Callaud. Another combination very closely resembling it, and giving nearly as favorable results, is known in this country as the Hill battery. In these elements the porous cup of the Daniell battery is entirely dispensed with, the two solutions being prevented from mingling by the difference of their respective specific gravities. The zinc plate of the Callaud element, in the form of a short hollow cylinder, open at both ends, is suspended in the upper portion of the containing jar, as shown in Fig. 56, by means of three hooks projecting from its upper edge, resting upon the jar. A strip of copper rolled into a spiral form is soldered to a copper wire covered with gutta-percha, forming the positive pole and connecting it to the zinc of the next element.
163. The manner of setting up this battery is as follows:
A sufficient quantity of soft water is poured into each jar to fill it to a point above the upper surface of the zinc. The battery should now be placed in the position which it is to permanently occupy, unless this has already been done. After the connections are made and everything in readiness, about three-quarters of a pound of sulphate of copper in lumps of the size of a hickory nut or larger, is dropped in, taking care that it does not lodge upon the zinc. The solution of sulphate of copper being of greater specific gravity, will remain at the bottom of the jar. The battery, after it is set up, should be kept on a closed circuit for about twelve hours, when its resistance will have become reduced so that the force will be available. As the battery continues in action, the sulphate of copper solution gradually becomes weaker and the zinc solution stronger. It is therefore necessary from time to time to add crystals of sulphate of copper, and to remove a portion of the zinc solution and replace by water. A good practical rule for maintaining this battery is to always see that the stratum of liquid around and in contact with the copper is kept of a blue color. The formation of transparent crystals upon the zinc indicates that the point of saturation of the zinc solution has been reached and that it should be diluted with water. A Baume hydrometer is very convenient for determining the density of the zinc solution. The latter should be maintained at from 20° to 30° in a main battery, and from 15° to 25° in a local.
It often occurs in using this battery that stalactites of copper attach themselves to the lower edge of the zinc and hang suspended in the solution, slowly but constantly increasing in length. These are first produced by a deposit of copper upon the zinc, which sets up a local action followed by a rapid decomposition of the solution and a further deposit of copper. These should be removed by means of a bent wire and allowed to fall to the bottom of the jar, as they occasion a useless expenditure of sulphate.
Absolute quietude is essential to the proper performance of this battery. A slight jar will cause the solutions to mingle, and this effect will be followed by a rapid deposition of metallic copper upon the zinc. When the zincs are removed for cleansing, care must be taken not to agitate the solution.
Prof. Hough, of the Dudley Observatory, has suggested the use of sheet lead in the place of the copper spiral, as it is cheaper and more readily cut and formed into proper shape. There is no perceptible difference in the electro-motive force or in the resistance of the battery when lead plates are substituted for copper in this way.
The electro-motive force of the gravity battery is the same as the Daniell, and the average resistance when in good working condition about three units.
164. Siemens' Universal Galvanometer.--- The apparatus employed for the measurement of electrical resistances consists essentially of a standard resistance, which is used for the purpose of comparison, a galvanometer, by which the result is indicated, and a galvanic battery. In the different methods of testing, these appliances are arranged in various ways, as particular circumstances may render convenient or desirable. The various methods of testing in use may be classified, however, under three heads, viz.:
1. By the angles of deflection of a galvanometer needle.
2. By the differential galvanometer.
3. By the Wheatstone bridge, or electrical balance.
The first-named method is the simplest in principle, and, with proper care, gives very accurate results. It is not so convenient as the other two methods for ordinary use, but is applicable more especially for the measurement of very high resistances, such as insulators, etc. It is also employed in measuring the internal resistance of batteries. As the strength of the current passing through the coils of a galvanometer is always proportionate to the sine or tangent of the angle of deflection of the needle, and is also inversely proportional to the resistance in circuit, it follows that if we find the deflection with a certain known resistance in circuit to be only 22°, and we then substitute for this known resistance an unknown one, which gives us a deflection of 39°, the tangent of the latter will be twice that of the former, and the unknown resistance is consequently found to be half that of the known resistance. (170.)
The second method is very convenient and is much used, although not equal in strict accuracy to the third method. The galvanometer coils are wound with two wires of the same length and resistance, insulated from each other with the utmost care. The needle is therefore surrounded by an equal number of convolutions of each wire, which are also equidistant from it. One end of each wire is connected to the battery, but in such a manner that the current flows in opposite directions through the wires. When, therefore, the two currents are of equal strength, one tends to deflect the needle to the right and the other to the left with equal power, and the needle remains at rest. If we insert an unknown resistance into the circuit of one of these wires the current is weakened, as is also its effect on the needle, which no longer remains balanced and at rest, but is deflected to one side. If we now insert a series of known resistances into the circuit of the other wire, until the needle is again brought into equilibrium, we are certain that the unknown resistance in one circuit is exactly equal to the known resistance in the other. (122.)
The third method is susceptible of the greatest accuracy of measurement, when proper precautions are observed. The connections of the ``bridge'' are arranged as follows:
We will suppose the wires A B C D (Fig. 57), arranged in the form of a parallelogram, to be of exactly equal resistance. If we attach the two poles of a battery, E, to the points 1 and 2, its current will divide at 1, half of it going through A B, and the other half going through C D, to the point 2, and thence to the other pole of the battery. The galvanometer G, placed on a wire connected across from 3 to 4, will not be affected as long as A B is equal to C D, no matter what the absolute resistance may be.
Again, when A bears the same proportion to C than B does to D,
or when A: C:: B: D, no current will pass from 3 to 4 through
the galvanometer. If the resistance of A be made 10, that of B
1, of C 1,000, and of D 100, the total resistance of A B will
now be 11, and that of C 1,100 ; but the tension in each branch
will have fallen in the same proportion at the points 3 and 4,
and no current will pass between those points.
If, therefore, we insert a known standard resistance in the wire
B, and an unknown one in D, and divide a given resistance
between A and C until we get no effect upon the galvanometer
needle, we are then certain that the resistance of A bears the
same proportion to that of B as the known resistance does to the
unknown one D, which may be readily calculated by proportion or
the ``rule of three.''
It is not necessary, of course, that the wires should be
arranged in the exact form shown, nor in fact is it often done,
but the principle is more easily explained and remembered when
thus arranged.
165. The Universal Galvanometer
of Dr. Werner Siemens is constructed upon the principle of the
Wheatstone bridge, just described, but its connections are so
arranged that it may be used when desired for the method of
deflections first mentioned.
The galvanometer is mounted upon a disk of slate about six
inches in diameter. A groove in the edge of this disk,
extending about half way round the circumference, contains a
wire of considerable resistance, which corresponds to the wires
A and C in the above diagram. A small platina roller, mounted
upon a radial arm, is connected to one pole of the battery, and
forms the connection with the wire A C, as shown at 1 in the
diagram. The wire corresponding to B is supplied with three
standard resistances of 10, 100, and 1,000 Siemens' units,
respectively, either of which may be placed in circuit at
pleasure, by means of contact plugs. The wire D is provided
with binding screws for the attachment of the wire, or other
resistance which it is required to measure. The galvanometer
consists of a pair of very delicate astatic needles, suspended
by a fine silk fibre. The coil has a resistance of 100 Siemens'
units.
The radial arm carrying the platina roller also carries a
pointer or index, moving over a scale upon the circumference of
the slate disk, which is divided into 300 degrees, and which may
be read to one-fifth of a degree by means of a vernier.
In using the instrument, the standard resistance corresponding
most nearly to the unknown resistance which is to be measured is
unplugged and placed in circuit at B (Fig. 57), while the
unknown resistance itself is inserted at D. The radial arm
carrying the platina roller 1 is now moved towards A or C, until
the needle is balanced. The proportion of A to C is then read
off the scale, from which the proportion of B to D is readily
calculated, or is taken from a printed table furnished with the
instrument.
This galvanometer may also be employed for comparing electro-
motive forces, according to the method of Poggendorff,* and is
applicable to almost any purpose for which an apparatus of the
kind may be required.
// footnote
* See Clark's ``Electrical Measurement,'' p. 105. Also Sabine's
``Elect. Tel.,'' p. 320.
// end footnote
This instrument usually has a constant of about four degrees,
with one Daniell's cell through 1,000,000 Siemens' units. When
used as a Wheatstone bridge, its range of measurement is from
0.17 to 59,000 Siemens' units. Higher resistances, such as
insulators, may be measured by the method of deflections. The
entire apparatus (except the battery) occupies a space only nine
inches in diameter, and the same in height. It is packed in a
neat case, and can be carried about with great convenience.
166. Pope & Edison's Printing
Telegraph.--- Type-printing telegraph instruments,
which were formerly employed for commercial telegraphing, have,
within two or three years, been extensively introduced, in a
modified and simple form, in the various branches of private
telegraphy, with great success. One of the best of these is
that of Pope & Edison, which is used on a large number of
private lines in New York city.
The different portions of the apparatus, with the exception of
the battery, are mounted upon a small table, similar in size and
construction to that of an ordinary sewing-machine. At the
back of the table are six binding screws to which are attached
the line and ground wires, and the wires leading to the main and
local batteries. The instrument operates upon what is known as
the ``open circuit principle,'' each station transmitting with
its own main battery---the line at the receiving station being
connected directly through the relay to the ground without the
intervention of a second battery.
The printing apparatus is placed upon a circular iron base in
the centre of the table. In front of it is placed a dial
containing the letters of the alphabet, arranged in a circle and
provided with an index or pointer, mounted upon a horizontal
shaft. This shaft also carries a type wheel, with the letters
of the alphabet engraved upon the type wheel and dial. An
electro-magnet beneath the base is provided with an armature,
attached to a vibrating lever, the latter armed with pawls or
clicks, so arranged in relation to the scape-wheel that every
time the electro-magnet attracts its armature, the wheel is
made to revolve a distance of one tooth, and the type wheel and
index upon the same shaft a distance of one letter. At the
extreme right of the circular base, and partly beneath it, as
seen in the engraving, is placed a second electromagnet, whose
armature lever passes in a horizontal direction below the type
wheel. Directly underneath the type wheel an india-rubber pad
is fixed upon the lever, by means of which an impression of the
letter which is opposite it upon the type wheel may be taken in
the manner hereafter to be described. This lever is also
provided with a simple mechanical device for moving the paper
forward the proper distance, as the impression of each
successive character is imprinted upon it. This may be seen at
the left of the printing apparatus. The type wheel is provided
with a suitable inking roller, as shown in the engraving.
It will thus be understood that the printing mechanism is
operated by two distinct electro-magnets, one of which is so
arranged that its successive pulsations may be made to advance
the index step by step to any required letter, while the other
forces the strip of paper against the inked type upon the wheel,
after it has been moved to the proper position by the first
magnet. The type wheel is, of course, so arranged in reference
to the index upon the same shaft, that when the latter points to
any given letter the corresponding letter upon the type wheel is
opposite the impression pad.
These two electro-magnets are placed in the circuit of a local
battery, which is brought into action by a relay placed in the
main line circuit, as in the ordinary Morse apparatus. The
relay is shown at the right of the printing mechanism, covered
by a small glass shade. It is the same in principle as the
ordinary Morse relay, with the addition of a device termed the
``polarized switch,'' which consists of a permanently magnetized
steel bar, pivoted between the poles of the relay magnet, and
forming a part of the local circuit. This is attracted to the
right or left according to the polarity of the relay magnet,
which itself, in turn, depends upon the direction of the
electrical current in the main circuit. The polarized switch
determines the direction of the local circuit, causing it to
pass through the magnet for moving the type wheel, or through
the impression magnet, as may be required.
Two lever finger keys, with vulcanite knobs, are placed on each
side of the printing apparatus, as shown in the engraving ; and
it is by means of these that the instrument is operated. They
are connected to opposite poles of the main battery in such a
manner that, by depressing the right hand key, the positive pole
of the battery is connected, through the relay magnet to the
line, and the negative to the ground, while the left hand key,
on the contrary, sends a negative current through the relay and
line in the same manner.
The mode of operating the instrument is exceedingly simple. By
depressing the right hand key a sufficient number of times in
rapid succession, a series of positive currents is sent through
the relays at both ends of the line, which are repeated upon the
local circuits of both instruments. The positive currents
deflect the polarized switches to the left, so that the local
circuit is directed into the type-wheel magnet. The index and
type wheel of both instruments, therefore, advance one letter
every time the key is depressed, and they may thus be readily
brought to any desired letter. When this has been done, the
left hand key is depressed, which sends a negative current,
reversing the polarized switch, and the local circuit is
directed through the printing magnet, producing the impression
of that letter upon the strip of paper, and this process may be
continued indefinitely.
Suitable arrangements are provided for bringing the type wheels
of the two instruments together in case they should accidentally
be thrown out of correspondence.
These instruments are entirely automatic in their action, and a
dispatch may be printed at the remote end of the line, in the
absence of an attendant. In the event of any derangement of the
printing apparatus, it may be used as a dial instrument as
conveniently as if especially constructed for that purpose.
The battery is always disconnected, except at the moment of
working, and therefore is consumed but slowly. Other systems
require the battery to be constantly connected to the line
whether working or idle. A battery of two carbon cells per mile,
and in many cases even less, will work the instrument and remain
in action from one to four weeks without renewal, according to
the amount of telegraphing done upon the line.
It is impossible for the main circuit to be accidentally left
open. Only one adjustment---that of the tension spring of the
relay---is required after the instrument is first put in
operation, and that but rarely on lines of ordinary length.
(Transcription note: The engraving referred to several times in
this section appeared nowhere in my copy of this book, which is
a pity, since it must have been a real beauty. In the sequence
of figures, number 58 is missing. There is no doubt that the
missing figure and the missing plate are one and the same, since
this section comes between sections that describe figures 57 and
59. If anyone finds a copy of this book with the missing
engraving, it would please me enormously to be able to complete
the catalog of figures.)![[IMAGE]](contacts/fig57-5.gif)