(_b_) The waxed rim cell, or cell No. 2, evaporated 26 ounces of water and the creeping of zinc sulphate salts was not prevented by the waxed rim. The wax proved of no value.
(_c_) The wax sealed cell, or cell No. 3, showed practically no evaporation and only very slight creeping of zinc sulphate salts.
The creeping of salts that took place was only around spots where the edges of the seal were loose from the jar.
(_d_) The paraffin oil sealed cell, or cell No. 4, showed no evaporation and no creeping of salts.
It was concluded by Mr. Reid from the above experiments that the wax applied to the rim of the jar is totally ineffective and has no merits. The wax seal loosens around the edges and does not totally prevent creeping of the zinc sulphate salts, although nearly so. The wax-sealed jar must have holes drilled in it to allow the gases to escape. The method is hardly commercial, as it is difficult to make a neat appearing cell, besides making it almost impossible to manipulate its contents. A coat of paraffin oil approximately 1/2 inch in thickness (about 6 ounces) gives perfect protection against evaporation and creeping of the zinc sulphate salts. The cell, having the paraffin-oil seal, had a very neat, clean appearance as compared with cells No. 1 and No. 2. It was found that the zinc could be drawn out through the oil, cleaned, and replaced with no appreciable effect on voltage or current.
Setting Up:--In setting up the battery the copper electrode is first unfolded to form a cross and placed in the bottom of the jar. Enough copper sulphate, or blue-stone crystals, is then dropped into the jar to almost cover the copper. The zinc crowfoot is then hung in place, occupying a position about 4 inches above the top of the copper. Clear water is then poured in sufficient to fill the jar within about an inch of the top.
If it is not required to use the cell at once, it may be placed on short circuit for a time and allowed to form its own zinc sulphate.
The cell may, however, be made immediately available for use by drawing about one-half pint of a solution of zinc sulphate from a cell already in use and pouring it into the jar, or, when this is not convenient, by putting into the liquid four or five ounces of pulverized sulphate of zinc, or by adding about ten drops of sulphuric acid. When the cell is in proper working condition, one-half inch in thickness of heavy paraffin oil of good quality may be added.
If the blue line gets too low, and if there is in the bottom of the cell a sufficient quant.i.ty of sulphate of copper, it may be raised by drawing off a portion of the zinc sulphate with a battery syringe and replacing this with water. If the blue line gets too high, it may be lowered by short-circuiting the cell for a time, or by the addition of more sulphate of zinc solution from another battery. If the copper sulphate becomes exhausted, it should be replenished by dropping in more crystals.
Care should be taken in cold weather to maintain the temperature of the battery above 65 or 70 Fahrenheit. If below this temperature, the internal resistance of a cell increases very rapidly, so much so that even at 50 Fahrenheit the action becomes very much impaired.
This follows from the facts that the resistance of a liquid decreases as its temperature rises, and that chemical action is much slower at lower temperatures.
The gravity cell has a practically constant voltage of 1.08 volts. Its internal resistance is comparatively high, seldom falling below 1 ohm and often rising to 6 ohms. At best, therefore, it is only capable of producing about 1 ampere. The gravity cell is perhaps the most common type of cell wherein depolarization is affected by electro-chemical means.
Fuller Cell:--A form of cell that is adapted to very heavy open-circuit work and also closed-circuit work where heavier currents are required than can be supplied by the gravity battery is the Fuller. In this the electrodes are of zinc and carbon, respectively, the zinc usually being in the form of a heavy cone and placed within a porous cup. The electrolyte of the Fuller cell is known as _electropoion fluid_, and consists of a mixture of sodium or pota.s.sium bichromate, sulphuric acid, and water.
The various parts of the standard Fuller cell, as once largely employed by the various Bell operating companies, are shown in Fig.
65. In this the jar was made of flint gla.s.s, cylindrical in form, six inches in diameter and eight inches deep. It is important that a good grade of gla.s.s be used for the jar in this cell, because, on account of the nature of the electrolyte, breakage is disastrous in the effects it may produce on adjacent property. The carbon plate is rectangular in form, about four inches wide, eight and three-quarters inches long, and one-quarter inch thick. The metal terminal at the top of the carbon block is of bronze, both it and the lock nuts and bolts being nickel-plated to minimize corrosion. The upper end of the carbon block is soaked in paraffin so hot as to drive all of the moisture out of the paraffin and out of the pores of the block itself.
The zinc, as is noted from the cut, is in the form of a truncated cone. It is about two and one-eighth inches in diameter at the base and two and one-half inches high. Cast into the zinc is a soft copper wire about No. 12 B. & S. gauge. This wire extends above the top of the jar so as to form a convenient terminal for the cell.
The porous cup is cylindrical in form, about three inches in diameter and seven inches deep. The wooden cover is of kiln-dried white wood thoroughly coated with two coats of asphalt paint. It is provided with a slot for the carbon and a hole for the copper wire extending to the zinc.
The electrolyte for this cell is made as follows:
Sodium bichromate 6 oz.
Sulphuric acid 17 oz.
Soft water 56 oz.
This solution is mixed by dissolving the bichromate of sodium in the water and then adding slowly the sulphuric acid. Pota.s.sium bichromate may be subst.i.tuted for the sodium bichromate.
In setting up this cell, the amalgamated zinc is placed within the porous cup, in the bottom of which are about two teaspoonfuls of mercury, the latter serving to keep the zinc well amalgamated. The porous cup is then placed in the gla.s.s jar and a sufficient quant.i.ty of the electrolyte is placed in the outer jar to come within about one and one-half inches of the top of the porous cup. About two teaspoonfuls of salt are then placed in the porous cup and sufficient soft water added to bring the level of the liquid within the porous cup even with the level of the electrolyte in the jar surrounding the cup. The carbon is then placed through the slot in the cover, and the wire from the zinc is pa.s.sed through the hole in the cover provided for it, and the cover is allowed to fall in place. The cell is now ready for immediate use.
The action of this cell is as follows: The sulphuric acid attacks the zinc and forms zinc sulphate, liberating hydrogen. The hydrogen attempts to pa.s.s to the carbon plate as usual, but in so doing it meets with the oxygen of the chromic acid and forms water therewith.
The remainder of the chromic acid combines with the sulphuric acid to form chromium sulphate.
[Ill.u.s.tration: Fig 65. Fuller Cell]
The mercury placed in the bottom of the porous cup with the zinc keeps the zinc in a state of perpetual amalgamation. This it does by capillary action, as the mercury spreads over the entire surface of the zinc. The initial amalgamation, while not absolutely essential, helps in a measure this capillary action.
In another well-known type of the Fuller battery the carbon is a hollow cylinder, surrounding the porous cup. In this type the zinc usually took the form of a long bar having a cross-shaped section, the length of this bar being sufficient to extend the entire depth of the porous cup. This type of cell has the advantage of a somewhat lower internal resistance than the standard form just described.
Should the electrolyte become supersaturated by virtue of the battery being neglected or too heavily overworked, a set of secondary reactions will occur in the cell, resulting in the formation of the yellow crystals upon the carbon. This seriously affects the e.m.f. of the cell and also its internal resistance. Should this occur, some of the solution should be withdrawn and dilute sulphuric acid inserted in its place and the crystals which have formed on the carbon should be carefully washed off. Should the solution lose its orange tint and turn blue, it indicates that more bichromate of potash or bichromate of sodium is needed. This cell gives an electromotive force of 2.1 volts and a very large current when it is in good condition, since its internal resistance is low.
The Fuller cell was once largely used for supplying current to telephone transmitters at subscribers" stations, where very heavy service was demanded, but the advent of the so-called common-battery systems, in some cases, and of the high-resistance transmitter, in other cases, has caused a great lessening in its use. This is fortunate as the cell is a "dirty" one to handle and is expensive to maintain.
The Fuller cell still warrants attention, however, as an available source of current, which may be found useful in certain cases of emergency work, and in supplying special but temporary needs for heavier current than the LeClanche or gravity cell can furnish.
Lalande Cell:--A type of cell, specially adapted to constant-current work, and sometimes used as a central source of current in very small common-battery exchanges is the so-called _copper oxide_, or _Lalande cell_, of which the Edison and the Gordon are types. In all of these the negatively charged element is of zinc, the positively charged element a ma.s.s of copper oxide, and the electrolyte a solution of caustic potash in water. In the Edison cell the copper oxide is in the form of a compressed slab which with its connecting copper support forms the electrode. In the Gordon and other cells of this type the copper oxide is contained loosely in a perforated cylinder of sheet copper. The copper oxide serves not only as an electrode, but also as a depolarizing agent, the liberated hydrogen in the electrolyte uniting with the oxygen of the copper oxide to form water, and leaving free metallic copper.
On open circuit the elements are not attacked, therefore there is no waste of material while the cell is not in use. This important feature, and the fact that the internal resistance is low, make this cell well adapted for all forms of heavy open-circuit work. The fact that there is no polarizing action within the cell makes it further adaptable to heavy closed-circuit service.
These cells are intended to be so proportioned that all of their parts become exhausted at once so that when the cell fails, complete renewals are necessary. Therefore, there is never a question as to which of the elements should be renewed.
After the elements and solution are in place about one-fourth of an inch of heavy paraffin oil is poured upon the surface of the solution in order to prevent evaporation. This cell requires little attention and will maintain a constant e.m.f. of about two-thirds of a volt until completely exhausted. It is non-freezable at all ordinary temperatures. Its low voltage is its princ.i.p.al disadvantage.
_Standard Cell_. Chloride of Silver Cell:--The chloride of silver cell is largely used as a standard for testing purposes. Its compactness and portability and its freedom from local action make it particularly adaptable to use in portable testing outfits where constant electromotive force and very small currents are required.
[Ill.u.s.tration: Fig. 66. Chloride of Silver Cell]
A cross-section of one form of the cell is shown in Fig. 66. Its elements are a rod of chemically-pure zinc and a rod of chloride of silver immersed in a water solution of sal ammoniac. As ordinarily constructed, the gla.s.s jar or tube is usually about 2-1/2 inches long by 1 inch in diameter. After the solution is poured in and the elements are in place the gla.s.s tube is hermetically sealed with a plug of paraffin wax.
The e.m.f. of a cell of this type is 1.03 volts and the external resistance varies with the age of the cell, being about 4 ohms at first. Care should be taken not to short-circuit these cells, or use them in any but high-resistance circuits, as they have but little energy and become quickly exhausted if compelled to work in low-resistance circuits.
Conventional Symbol. The conventional symbol for a cell, either of the primary or the secondary type, consists of a long thin line and a short heavy line side by side and parallel. A battery is represented by a number of pairs of such lines, as in Fig. 67. The two lines of each pair are supposed to represent the two electrodes of a cell.
Where any significance is to be placed on the polarity of the cell or battery the long thin line is supposed to represent the positively charged plate and the short thick line the negatively charged plate.
The number of pairs may indicate the number of cells in the battery.
Frequently, however, a few pairs of such lines are employed merely for the purpose of indicating a battery without regard to its polarity or its number of cells.
[Ill.u.s.tration: Fig. 67. Battery Symbols]
In Fig. 67 the representation at _A_ is that of a battery of a number of cells connected in parallel; that at _B_ of a battery with the cells connected in series; and that at _C_ of a battery with one of its poles grounded.
CHAPTER VIII
MAGNETO SIGNALING APPARATUS
Method of Signaling. The ordinary apparatus, by which speech is received telephonically, is not capable of making sufficiently loud sounds to attract the attention of people at a distance from the instrument. For this reason it is necessary to employ auxiliary apparatus for the purpose of signaling between stations. In central offices where an attendant is always on hand, the sense of sight is usually appealed to by the use of signals which give a visual indication, but in the case of telephone instruments for use by the public, the sense of hearing is appealed to by employing an audible rather than a visual signal.
Battery Bell. The ordinary vibrating or battery bell, such as is employed for door bells, is sometimes, though not often, employed in telephony. It derives its current from primary batteries or from any direct-current source. The reason why they are not employed to a greater extent in telephony is that telephone signals usually have to be sent over lines of considerable length and the voltage that would be required to furnish current to operate such bells over such lengths of line is higher than would ordinarily be found in the batteries commonly employed in telephone work. Besides this the make-and-break contacts on which the, ordinary battery bell depends for its operation are an objectionable feature from the standpoint of maintenance.
Magneto Bell. Fortunately, however, there has been developed a simpler type of electric bell, which operates on smaller currents, and which requires no make-and-break contacts whatever. This simpler form of bell is commonly known as the _polarized_, or _magneto_, bell or _ringer_. It requires for its operation, in its ordinary form, an alternating current, though in its modified forms it may be used with pulsating currents, that is, with periodically recurring impulses of current always in the same direction.
Magneto Generator. In the early days of telephony there was nearly always a.s.sociated with each polarized bell a magneto generator for furnishing the proper kind of current to ring such bells. Each telephone was therefore equipped, in addition to the transmitter and receiver, with a signal-receiving device in the form of a polarized bell, and with a current generator by which the user was enabled to develop his own currents of suitable kind and voltage for ringing the bells of other stations.
Considering the signaling apparatus of the telephones alone, therefore, each telephone was equipped with a power plant for generating currents used by that station in signaling other stations, the prime mover being the muscles of the user applied to the turning of a crank on the side of the instrument; and also with a current-consuming device in the form of a polarized electromagnetic bell adapted to receive the currents generated at other stations and to convert a portion of their energy into audible signals.
The magneto generator is about the simplest type of dynamo-electric machine, and it depends upon the same principles of operation as the much larger generators, employed in electric-lighting and street-railway power plants, for instance. Instead of developing the necessary magnetic field by means of electromagnets, as in the case of the ordinary dynamo, the field of the magneto generator is developed by permanent magnets, usually of the horseshoe form. Hence the name _magneto_.