QUESTIONS AND PROBLEMS 1. Describe and explain the electrolysis of water, covering the following points: (1) ionization of the acid added to the water; (2) material of the anode; (3) electrical state of the anode and the cathode; (4) cause of ion migration; (5) circumstances accompanying the evolution of oxygen. 2. 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? 3. How could a silver cup be given a gold lining by use of the electric current? 4. How many ampere hours are required to deposit 16.1 g. of silver? How strong was the current that did this, if the deposit occurred in 1 hr.? in 2 hr.? in hr.? 5. If the electrochemical equivalent of silver (expressed as grams per second per ampere) is .001118, how long will it take to deposit 2.4 grams of silver on a spoon if 3 amperes of current are used? 6. What was the strength of a current that deposited 11.84 g. of copper in 30 min. ? 7. How many coulombs in an ampere hour? 8. How many ampere hours in a coulomb? MAGNETIC EFFECTS OF THE CURRENT; PROPERTIES OF COILS 307. 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. 253) 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 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. 253). If the current is weak, the experiment should be performed by using a large number of turns of wire, as indicated in Fig. 254. 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 Ampère's rule: If the right hand grasps the wire as in 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. 308. Loop of wire carrying a current equivalent to a magnet disk. Let a single loop of wire be suspended from a thread in the manner shown in Fig. 256, so that its ends dip into two mercury cups. Then let the current from three or four dry cells be sent through the loop. The latter will be found slowly to set itself so that the face of the loop from which the magnetic lines emerge, as given by the right-hand rule (see § 307 and also Fig. 257), is toward the north. Let a bar magnet be brought near FIG. 256. A loop equivalent to a flat magnetic disk the loop. The latter will be found to behave toward the magnet in all respects as though it were a flat magnetic disk whose boundary is the wire, the face which turns toward the north being an N pole and the other an S pole. The experiment shows what position a loop bearing a current will always tend to assume in a magnetic field; for, since a magnet will always tend to set itself so that the line connecting its poles is parallel to the direction of the magnetic lines of the field in which it is placed, a loop must set itself so that a line connecting its magnetic poles is parallel to the lines of the magnetic field, that is, so that the plane of the loop is perpendicular to the field (see Fig. 258); or, to FIG. 257. North pole of disk is face from which FIG. 258. Position assumed by a loop carrying a current in a magnetic field state the same thing in slightly different form, if a loop of wire, free to turn, is carrying a current in a magnetic field, the loop will set itself so as to include as many as possible of the lines of force of the field. 309. Helix carrying a current equivalent to a bar magnet. Let a wire bearing a current be wound into a series of loops (a helix) and held near a suspended magnet, as in Fig. 259. It will be found to act in every respect like a magnet, with an N pole at one end and an S pole at the other. This result might have been predicted from the fact that a single loop is equivalent to a flat-disk magnet; for when such disks are placed side by side in a series, as in the helix, the result must be the same as placing a series of disk magnets in a row, the N pole of one being directly in contact with the S pole of the next, etc. These poles would therefore alì neutralize each other except at the two ends. We therefore get a magnetic field of the shape shown in Fig. 260, the direction of the arrows representing as usual the direction in which an N pole tends to move. The right-hand rule as given in § 307 is sufficient in every case to determine which is the N and which the S pole of a N FIG. 259. Magnetic effect of a helix FIG. 260. Magnetic field of helix N -S helix, that is, from which end the lines of magnetic force emerge from the helix and at which end they enter it. But it is found convenient, in the consideration of coils, to restate the right-hand rule in a slightly different way, thus: If the coil is grasped in the right hand in such a way that the fingers point in the direction in which the current is flowing in the wires, the thumb will point in the direction of the north pole of the helix (see Fig. 261). Similarly, if the sign of the poles is known, but the direction of the current unknown, it may be determined as follows: If the right hand is placed against the coil with the thumb pointing in the direction of the lines of force (that is, toward the north pole of the helix), the fingers will pass around the coil in the direction in which the current is flowing. Direction of current FIG. 261. Rule for poles of helix 310. The electromagnet. Let a core of soft iron be inserted in the helix (Fig. 262). The poles will be found to be enormously stronger than before. This is because the core is magnetized by induction from the field of the helix in precisely the same way in which it would be magnetized by induction if placed in the field of a permanent magnet. The new field strength about the coil is now the sum of the fields due to the core and that due to the coil. If the current is broken, the core will at once lose the greater part of its magnetism. If the current is reversed, the polarity of the core will be reversed. Such a coil with a soft-iron core is called an electromagnet. The strength of an electromagnet can be very greatly increased by giving it such form that the magnetic lines can FIG. 262. The bar electro- FIG. 263. The horseshoe electromagnet remain in iron throughout their entire length instead of emerging into air, as they do in Fig. 262. For this reason electromagnets are usually built in the horseshoe form and provided with an armature A (Fig. 263), through which a complete iron path for the lines of force is established, as shown in Fig. 264. The strength of such a magnet depends upon the quantity and quality and form of the iron, but chiefly upon the number of ampere turns which encircle it, the expression 'ampere turns" denoting the product of the number of turns of wire about the magnet by the number of amperes flowing in each turn. Thus, a current of Tampere flowing 1000 times around a core will make an electromagnet of precisely the same strength as a current of 1 ampere flowing 10 times about the core. (See modern lifting magnet opposite page 265.) ९९ FIG. 264. The magnetic circuit of an electromagnet |