HOW ELECTRICITY MAY BE MEASURED
312. Danger of an Oversupply of Current. If a small toy motor is connected with one cell, it rotates slowly; if connected with two cells, it rotates more rapidly, and in general, the greater the number of cells used, the stronger will be the action of the motor. But it is possible to send too strong a current through our wire, thereby interfering with all motion and destroying the motor. We have seen in Section 288 that the amount of current which can safely flow through a wire depends upon the thickness of the wire. A strong current sent through a fine wire has its electrical energy transformed largely into heat; and if the current is very strong, the heat developed may be sufficient to burn off the insulation and melt the wire itself. This is true not only of motors, but of all electric machinery in which there are current-bearing wires. The current should not be greater than the wires can carry, otherwise too much heat will be developed and damage will be done to instruments and surroundings.
The current sent through our electric stoves and irons should be strong enough to heat the coils, but not strong enough to melt them. If the current sent through our electric light wires is too great for the capacity of the wires, the heat developed will injure the wires and may cause disastrous results. The overloading of wires is responsible for many disastrous fires.
The danger of overloading may be eliminated by inserting in the circuit a fuse or other safety device. A fuse is made by combining a number of metals in such a way that the resulting substance has a low melting point and a high electrical resistance. A fuse is inserted in the circuit, and the instant the current increases beyond its normal amount the fuse melts, breaks the circuit, and thus protects the remaining part of the circuit from the danger of an overload. In this way, a circuit designed to carry a certain current is protected from the danger of an accidental overload. The noise made by the burning out of a fuse in a trolley car frequently alarms passengers, but it is really a sign that the system is in good working order and that there is no danger of accident from too strong a current.
313. How Current is Measured. The preceding Section has shown clearly the danger of too strong a current, and the necessity for limiting the current to that which the wire can safely carry. There are times when it is desirable to know accurately the strength of a current, not only in order to guard against an overload, but also in order to determine in advance the mechanical and chemical effects which will be produced by the current. For example, the strength of the current determines the thickness of the coating of silver which forms in a given time on a spoon placed in an electrolytic bath; if the current is weak, a thin plating is made on the spoon; if the current is strong, a thick plating is made. If, therefore, the exact value of the current is known, the exact amount of silver which will be deposited on the spoon in a given time can be definitely calculated.
FIG. 233.—The principle of the galvanometer.
Current-measuring instruments, or galvanometers, depend for their action on the magnetic properties of current electricity. The principle of practically all galvanometers is as follows:—
A closely wound coil of fine wire free to rotate is suspended as in Figure 233 between the poles of a strong magnet. When a current is sent through the coil, the coil becomes a magnet and turns so that its faces will be towards the poles of the permanent magnet. But as the coil turns, the suspending wire becomes twisted and hinders the turning. For this reason, the coil can turn only until the motion caused by the current is balanced by the twist of the suspending wire. But the stronger the current through the coil, the stronger will be the force tending to rotate the coil, and hence the less effective will be the hindrance of the twisting string. As a consequence, the coil swings farther than before; that is, the greater the current, the farther the swing. Usually a delicate pointer is attached to the movable coil and rotates freely with it, so that the swing of the pointer indicates the relative values of the current. If the source of the current is a gravity cell, the swing is only two thirds as great as when a dry cell is used, indicating that the dry cell furnishes about 1-1/2 times as much current as a gravity cell.
314. Ammeters. A galvanometer does not measure the current, but merely indicates the relative strength of different currents. But it is desirable at times to measure a current in units. Instruments for measuring the strength of currents in units are called ammeters, and the common form makes use of a galvanometer.
A current is sent through a movable coil (the field magnet and coil are inclosed in the case) (Fig. 234), and the magnetic field thus developed causes the coil to turn, and the pointer attached to it to move over a scale graduated so that it reads current strengths. This scale is carefully graduated by the following method.
If two silver rods (Fig. 208) are weighed and placed in a solution of silver nitrate, and current from a single cell is passed through the liquid for a definite time, we find, on weighing the two rods, that one has gained in weight and the other has lost. If the current is allowed to flow twice as long, the amount of silver lost and gained by the electrodes is doubled; and if twice the current is used, the result is again doubled.
FIG. 234.—An ammeter.
As a result of numerous experiments, it was found that a definite current of electricity will deposit a definite amount of silver in a definite time, and that the amount of silver deposited on an electrode in one second might be used to measure the current of electricity which has flowed through the circuit in one second.
A current is said to be one ampere strong if it will deposit silver on an electrode at the rate of 0.001118 gram per second.
In marking the scale, an ammeter is placed in the circuit of an electrolytic cell and the position of the pointer is marked on the blank card which lies beneath and which is to serve as a scale (Fig. 235). After the current has flowed for about an hour, the amount of silver which has been deposited is measured. Knowing the time during which the current has run, and the amount of deposit, the strength of the current in amperes can be calculated. This number is written opposite the place at which the pointer stood during the experiment.
The scale may be completed by marking the positions of the pointer when other currents of known strength flow through the ammeter.
FIG. 235.—Marking the scale of an ammeter.
All electric plants, whether for heating, lighting, or for machinery, are provided with ammeters, such instruments being as important to an electric plant as the steam gauge is to the boiler.
315. Voltage and Voltmeters. Since electromotive force, or voltage, is the cause of current, it should be possible to compare different electromotive forces by comparing the currents which they produce in a given circuit. But two voltages of equal value do not give equal currents unless the resistances met by the currents are equal. For example, the simple voltaic cell and the gravity cell have approximately equal voltages, but the current produced by the voltaic cell is stronger than that produced by the gravity cell. This is because the current meets more resistance within the gravity cell than within the voltaic cell. Every cell, no matter what its nature, offers resistance to the flow of electricity through it and is said to have internal resistance. If we are determining the voltages of various cells by a comparison of the respective currents produced, the result will be true only on condition that the resistances in the various circuits are equal. If a very large external resistance of fine wire is placed in circuit with a gravity cell, the total resistance of the circuit (made up of the relatively small resistance in the cell and the larger resistance in the rest of the circuit) will differ but little from that of another circuit in which the gravity cell is replaced by a voltaic cell, or any other type of cell.
With a high resistance in the outside circuit, the deflections of the ammeter will be small, but such as they are, they will fairly accurately represent the electromotive forces which produce them.
Voltmeters (Fig. 236), or instruments for measuring voltage, are like ammeters except that a wire of very high resistance is in circuit with the movable coil. In external appearance they are not distinguishable from ammeters.
FIG. 236.—A voltmeter.
The unit of electromotive force is called the volt. The voltage of a dry cell is approximately 1.5 volts, and the voltage of a voltaic cell and of a gravity cell is approximately 1 volt.
316. Current, Voltage, Resistance. We learned in Section 287 that the strength of a current increases when the electromotive force increases, and diminishes when the electromotive force diminishes. Later, in Section 288, we learned that the strength of the current decreases as the resistance in circuit increases.
The strength of a steady current depends upon these two factors only, the electromotive force which causes it and the resistance which it has to overcome.
317. Resistance. Since resistance plays so important a rôle in electricity, it becomes necessary to have a unit of resistance. The practical unit of resistance is called an ohm, and some idea of the value of an ohm can be obtained if we remember that a 300-foot length of common iron telegraph wire has a resistance of 1 ohm. An approximate ohm for rough work in the laboratory may be made by winding 9 feet 5 inches of number 30 copper wire on a spool or arranging it in any other convenient form.
In Section 299 we learned that substances differ very greatly in the resistance which they offer to electricity, and so it will not surprise us to learn that while it takes 300 feet of iron telegraph wire to give 1 ohm of resistance, it takes but 39 feet of number 24 copper wire, and but 2.2 feet of number 24 German silver wire, to give the same resistance.
NOTE. The number of a wire indicates its diameter; number 30, for example, being always of a definite fixed diameter, no matter what the material of the wire.
If we wish to avoid loss of current by heating, we use a wire of low resistance; while if we wish to transform electricity into heat, as in the electric stove, we choose wire of high resistance, as German silver wire.