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such as a short length of a good conductor; others have so much as to form a most effectual barrier to the passage of the current, these being commonly known as insulators. As an example, consider a man lifting weights. The heavier the weight, the harder he must work to lift it. A little body weighing a few ounces offers so little resistance that it can be raised from the ground with a negligible amount of work. At the other hand it may have a mass
of several tons, in which case enough resistance would be offered to make it immovable against the efforts of one man, though a number of men might easily move it without mechanical aid.
A substance that would offer considerable resistance to a current of low-tension or voltage would be easily overcome by a current having greater electro-motive force. For instance, it is impossible to pass the current obtained, from several cells of dry battery through the air gap between the points of a plug, as the current
pressure, only a few volts, is not sufficient to overcome the resistance offered by the air between the electrodes. At the other hand, pass this same battery current through a transformer, such as an induction coil, and it produces another current of greater potential and at high enough voltage to overcome the resistance of the air gap. As an example a column of mercury one square millimeter in cross section and 1.0624 meters long has the resistance of one ohm.
The unit of current strength or intensity is the AMPERE. Take a conductor of one ohm resistance, maintain a difference of potential of one volt between the ends, and in one second a quantity of current equal to one ampere would have passed through it. A greater voltage will maintain a greater current flow through the same resistance, or a greater resistance will reduce the current in exact proportion. For example, two volts would pass two amperes through one ohm resistance but would only pass one ampere through two ohms resistance. The ampere is really a unit denoting the rate of flow, and is exactly analogous to a well-known unit used in hydraulics, known as the “miner's inch.” This denotes the rate of flow of water which, under a head of six inches, will pass through a hole one inch square in a board two inches thick. Let this head of water represent one volt, and let the resistance of the hole represent one ohm, then the “miner's inch” would represent a current of an ampere.
A WATT is a unit of quantity, or amount of electric energy, and corresponds to a current of one ampere at a pressure of one volt. There are other terms in which a unit of time is compounded with the foregoing, such as ampere-hour, which means that a generator of certain capacity could maintain a current of one ampere for one hour. It is a term usually applied to determine the capacity of a chemical producer. For instance, a storage battery with 60 ampere-hours capacity should supply, theoretically, a current of one ampere for 60 hours, two amperes for 30 hours or 60 amperes for one hour, or any other combination of time and amperes which would produce the same result. Electrical rate of work is measured by a unit involving potential difference, quantity of electricity and time. Thus a watt is a volt-ampere-second, and 746
watts indicate an amount of electrical energy equal to one mechanical horsepower.
Electrical Measuring Instruments.-As the electric force is intangible and is known only by its effects, it is necessary to have methods of measuring the amount employed to properly use the current. If the current was too strong injurious results might follow and if not strong enough satisfactory results could not be secured. The electric force can be measured by relatively simple devices. Most of the electrical measuring instruments depend upon the principle of electro-magnetism or induction and may be classified as moving iron, moving coil, solenoid and plunger, magnetic vane, hot wire, inclined coil, etc. The four first named are the most commonly used in measuring the current employed in starting and lighting systems. These measuring instruments are made in portable and switchboard types. The windings in an instrument designed to measure current quantity or amperage is usually of coarse wires, while the windings of an instrument to measure electro motive force or voltage will be of finer wire. The gauge used to measure current quantity is called an ampere meter or ammeter while that used to measure current pressure is a volt meter.
The various forms of electrical measuring instruments and the method of operation may be readily understood by referring to the illustrations at Fig. 21. The instrument shown at A is known as a moving iron type. In this a permanent magnet holds a soft iron indicator to which the pointer needle is attached so that it registers with zero on the scale until a current passes through the coil and the magnetic lines of force thus produced tend to pull the needle in line with them and thereby actuate the pointer. The movement of the soft iron indicator depends entirely upon the amount of current passing through the coil. The moving coil pipe which is shown at B is the most popular form, as it gives the most reliable indication. The parts of a complete instrument of this form are clearly outlined at Fig. 22. This consists of a permanent magnet carrying a fixed pole piece about which a small solenoid capable of oscillating back and forth on jeweled bearings is mounted. On the cheap instruments ordinary pivot bearings are used instead of the jewels. A hand or pointer is pivoted at the
Fig. 21.-Diagrams Outlining Construction of Common Types of Current
bearing and moves with the coil to indicate the variations of current strength on a graduated scale with which the pointer registers. A steel hair spring attached to the coil acts to restrain and control its movements and to return the pointer to zero when the current
ceases to flow through the solenoid. The function of the magnetic field is to keep the solenoid steady, though as soon as an electric current passes through its equilibrium is upset and the degree of movement is proportional to the amount or pressure of the current passing through it. Many small instruments which are accurate and inexpensive have been devised for testing current strength.
For convenience the mechanism has been enclosed in standard watch movement cases in many instances.
The plunger type of indicator which is shown at C and D operates on the principle of attraction that a solenoid exerts upon materials susceptible to its influence. A curved plunger is used in that type usually intended for switch-board use. When a current is passed through the solenoid, the plunger is drawn into the interior of the coil, the amount of movement depending upon the current strength. This is indicated by
This is indicated by a calibrated scale and ·