« PreviousContinue »
upon the needle, we learn that although the cell develops a very small P.D. between its terminals, it nevertheless sends through the connecting wire very much more electricity per second than the static machine is able to send. This is because the chemical action of the cell is able to recharge the plates to their small P.D. practically as fast as they are discharged through the wire, whereas the static machine requires a relatively long time to recharge its terminals to their high P.D. after they have once been discharged.
QUESTIONS AND PROBLEMS
1. Under what conditions will an electric charge produce a magnetic effect?
2. How can you test whether or not a current is flowing in a wire? 3. How does the current delivered by a cell differ from that delivered by a static machine?
4. Mention three respects in which the behavior of magnets is similar to that of electric charges; two respects in which it is different.
CHEMICAL EFFECTS OF THE CURRENT; ELECTROLYSIS *
302. Electrolysis. Let two platinum electrodes be dipped into a solution of dilute sulphuric acid, and let the terminals of a battery producing a pressure of 10 volts or more be applied to these electrodes. Oxygen gas is found to be given off at the electrode at which the current enters the solution, called the anode, while hydrogen is given off at the electrode at which the current leaves the solution, called the cathode. These gases may be collected in test tubes in the manner shown in Fig. 245.
In accordance with the theory now in vogue among physicists and chemists, when sulphuric acid is mixed with water so as to form a dilute solution, the H2SO molecules split up into three electrically charged parts, called ions, the two
*This subject should be accompanied or followed by a laboratory experiment on electrolysis and the principle of the storage battery. See, for example, Experiment 35 of the authors' Manual.
hydrogen ions each carrying a positive charge and the SO ion a double negative charge (Fig. 246). This phenomenon is known as dissociation. The solution as
a whole is neutral; that is, it is uncharged, because it contains just as many positive as negative charges.
As soon as an electrical field is established in the solution by connecting the electrodes to the positive and negative terminals of a battery, the hydrogen ions begin to migrate toward the negative elec
FIG. 245. Electrolysis of water
trode (that is, the cathode) and there, after giving up their charges, unite to form molecules of hydrogen gas (Fig. 245). On the other hand, the negative
FIG. 246. Showing dissociation of sulphuric-acid molecules in water
and of oxygen are measured, the hydrogen is found to occupy in every case just twice the volume occupied by the oxygen. This is, indeed, one of the reasons for believing that a molecule of water consists of two atoms of hydrogen and one of oxygen.
303. Electroplating. If the solution, instead of being sulphuric acid, had been one of copper sulphate (CuSO), the results would have been precisely the same in every respect, except that, since the hydrogen ions in the solution are now replaced by copper ions, the substance deposited on the cathode is pure copper instead of hydrogen. This is the principle involved in electroplating of all kinds. In commercial work the positive plate, that is, the plate at which the current
enters the bath, is always made from the same metal as that which is to be deposited from the solution, for in this case the SO, or other negative ions dissolve this plate as fast as the metal ions are deposited upon
FIG. 247. A simple electroplating bath
the other. The strength of the solution, therefore, remains unchanged. In effect, the metal is simply taken from one plate and deposited on the other. Fig. 247 represents a simple form of silver-plating bath. The anode A is of pure silver. The spoon to be plated is the cathode K. In practice the articles to be plated are often suspended from a central rod (Fig. 248), while on both sides about the articles are the suspended anodes. This arrangement gives
a more even deposit of metal. In silver plating the solution consists of 500 grams of potassium cyanide and 250 grams of silver cyanide in 10 liters of
FIG. 248. Electroplating bath
304. Electrotyping. In the process of electrotyping, the page is first set up in the form of common type. A mold is then taken in wax or gutta-percha. This mold is then coated with powdered graphite to render it a conductor, after which it is ready to be suspended as the cathode in a copper-plating bath, the anode being a plate of pure copper and the liquid a solution of copper sulphate. When a sheet of copper as thick as a visiting card has been deposited on the mold, the latter is removed and the wax replaced by a type-metal backing, to give rigidity to the copper films. From such a plate as many as a hundred thousand impressions may be made. Nearly all books which run through large editions are printed from such electrotypes.
305. Legal units of current and quantity. In 1834 Faraday (see opposite p. 290) found that a given current of electricity flowing for a given time always deposits the same amount of a given element from a solution, whatever be the nature of the solution which contains the element. For example, one ampere, the unit of current, always deposits in an hour 4.025 grams of silver, whether the electrolyte is silver nitrate, silver cyanide, or any other silver compound. Similarly, an ampere will deposit in an hour 1.181 grams of copper, 1.203 grams of zinc, etc. Faraday further found that the amount of metal deposited in a given cell depended solely on the product of the current strength by the time, that is, on the quantity of electricity which had passed through the cell. These facts are made the basis of the legal definitions of current and quantity, thus:
The unit of quantity, called the coulomb, is the quantity of electricity required to deposit .001118 gram of silver.
The unit of current, the ampere, is the current which will deposit .001118 gram of silver in one second.
QUESTIONS AND PROBLEMS
1. What was the strength of a current that deposited 11.84 g. of copper in 30 min. ?
2. How long will it take a current of 1 ampere to deposit 1 g. of silver from a solution of silver nitrate?
3. If the same current used in Problem 2 were led through a solution containing a zinc salt, how much zinc would be deposited in the same time? 4. How could a silver cup be given a gold lining by use of the electric current?
5. If the terminals of a battery are immersed in a glass of acidulated water, how can you tell from the rate of evolution of the gases at the two electrodes which is positive and which is negative?
6. The coulomb (§ 305) is 3 billion times as large as the electrostatic unit of quantity defined in § 280. How many electrons pass per second by a given point on a lamp filament which is carrying 1 ampere of current (see § 284)?
MAGNETIC EFFECTS OF THE CURRENT; PROPERTIES
306. Shape of the magnetic field about a current. If we place the wire which connects the plates of a galvanic cell in a vertical position (Fig. 249) and explore with a compass needle the shape of the magnetic field about the current, we find that the magnetic lines are concentric circles lying in a plane perpendicular to the wire and having
Magnetic field about a current
the wire as their common center. We find, moreover, that reversing the current reverses the direction of the needle. If the current is very strong (say 40 amperes), this shape of the field can be shown by scattering iron filings on a plate through which the current passes (Fig. 249). If the current is weak, the experiment should be performed as indicated in Fig. 250.
FIG. 251. The right-hand rule
The relation between the direction in which the current flows and the direction in which the N pole of the needle points (this is, by definition, the direction of the magnetic field) is given in the following convenient rule, known as Ampere's Rule: If the right hand grasps the wire as in Fig. 251, so that the thumb points in the direction in which the current is flowing, then the magnetic lines encircle the wire in the same direction as do the fingers of the hand.