Chapter VI

UNITS AND APPARATUS FOR ELECTRICAL MEASUREMENTS.

=64. Electrical Units.= In order to measure electricity for experimental or commercial purposes, standards or units are just as necessary as the inch or foot for measuring distances.

=65. Potential; Electromotive Force.= If water in a tall tank be allowed to squirt from two holes, one near the bottom, the other near the top, it is evident that the force of the water that comes from the hole at the bottom will be the greater. The pressure at the bottom is greater than that near the top, because the "head" is greater.

When a spark of static electricity jumps a long distance, we say that the charge has a high _potential_; that is, it has a high electrical pressure. Potential, for electricity, means the same as pressure, for water. The greater the potential, or _electromotive force_ (E.M.F.) of a cell, the greater its power to push a current through wires. (See "Study," § 296 to 305, with experiments.)

=66. Unit of E.M.F.; the Volt.=--In speaking of water, we say that its pressure is so many pounds to the square inch, or that it has a fall, or head, of so many feet. We speak of a current as having so many volts; for example, we say that a wire is carrying a 110-volt current. The volt is the unit of E.M.F. An ordinary gravity cell has an E.M.F. of about one volt. This name was given in honor of Volta.

=67. Measurement of Electromotive Force.= There are several ways by which the E.M.F. of a cell, for example, can be measured. It is usually measured _relatively_, by comparison with the E. M. F. of some standard cell. (See "Study," Exp. 140, for measuring the E. M. F. of a cell by comparison with the two-fluid cell.)

_Voltmeters_ are instruments by means of which E. M. F. can be read on a printed scale. They are a variety of galvanometer, and are made with coils of such high resistance, compared with the resistance of a cell or dynamo, that the E. M. F. can be read direct. The reason for this will be seen by referring to Ohm's law ("Study," § 356); the resistance is so great that the strength of the current depends entirely upon the E. M. F.

Voltmeters measure electrical pressure just as steam gauges measure the pressure of steam. Fig. 62 shows one form of voltmeter. Fig. 63 shows a voltmeter with illuminated dial. An electrical bulb behind the instrument furnishes light so that the readings can be easily taken.

=68. Electrical Resistance.= Did you ever ride down hill on a hand-sled? How easily the sled glides over the snow! What happens, though, when you strike a bare place, or a place where some evil-minded person has sprinkled ashes? Does the sled pass easily over bare ground or ashes? Snow offers very little _resistance_ to the sled, while ashes offer a great resistance.

All substances do not allow the electric current to pass through them with the same ease. Even the liquid in a cell tends to hold the current back and offers _internal resistance_. The various wires and instruments connected to a cell offer _external resistance_. (See "Study," Chapter XVIII., for experiments, etc.)

=69. Unit of Resistance.= =The Ohm= is the name given to the unit of resistance. About 9 ft. 9 in. of No. 30 copper wire, or 39 feet 1 in. of No. 24 copper wire, will make a fairly accurate ohm.

_Resistance coils_, having carefully measured resistances, are made for standards. (See "Apparatus Book," Chapter XVII., for home-made resistance coils.) Fig. 64 shows a commercial form of a standard resistance coil. The coil is inclosed in a case and has large wires leading from its ends for connections. Fig. 65 gives an idea of the way in which coils are wound and used with plugs to build up _resistance boxes_, Fig. 66.

=70. Laws of Resistance.= 1. The resistance of a wire is directly proportional to its length, provided its cross-section, material, etc., are uniform.

2. The resistance of a wire is inversely proportional to its area of cross-section; or, in other words, inversely proportional to the square of its diameter, other things being equal.

3. The resistance of a wire depends upon its material, as well as upon its length, size, etc.

4. The resistance of a wire increases as its temperature rises. (See "Study," Chapters XVIII. and XIX., for experiments on resistance, its measurement, etc.)

=71. Current Strength.= The strength of a current at the end of a circuit depends not only upon the _electrical pressure_, or E. M. F., which drives the current, but also upon the _resistance_ which has to be overcome. The greater the resistance the weaker the current at the end of its journey.

=72. Unit of Current Strength; The Ampere.= A current having an E. M. F. of _one volt_, pushing its way through a resistance of _one ohm_, would have a unit of strength, called _one ampere_. This current, one ampere strong, would deposit, under proper conditions, .0003277 gramme of copper in _one second_ from a solution of copper sulphate.

=73. Measurement of Current Strength.= A magnetic needle is deflected when a current passes around it, as in instruments like the galvanometer. The _galvanoscope_ merely indicates the presence of a current. _Galvanometers_ measure the strength of a current, and they are made in many forms, depending upon the nature and strength of the currents to be measured. Galvanometers are standardized, or calibrated, by special measurements, or by comparison with some standard instrument, so that when the deflection is a certain number of degrees, the current passing through it is known to be of a certain strength.

Fig. 67 shows an _astatic galvanometer_. Fig. 68 shows a _tangent galvanometer_, in which the strength of the current is proportional to the tangent of the angle of deflection. Fig. 69 shows a _D'Arsonval galvanometer_, in which a coil of wire is suspended between the poles of a permanent horseshoe magnet. The lines of force are concentrated by the iron core of the coil. The two thin suspending wires convey the current to the coil. A ray of light is reflected from the small mirror and acts as a pointer as in other forms of reflecting galvanometers.

=74. The Ammeter=, Fig. 70, is a form of galvanometer in which the strength of a current, in amperes, can be read. In these the strength of current is proportional to the angular deflections. The coils are made with a small resistance, so that the current will not be greatly reduced in strength in passing through them.

=75. Voltameters= measure the strength of a current by chemical means, the quantity of metal deposited or gas generated being proportional to the time that the current flows and to its strength. In the _water voltameter_, Fig. 71, the hydrogen and oxygen produced in a given time are measured. (See "Study," Chapter XXI.)

The _copper voltameter_ measures the amount of copper deposited in a given time by the current. Fig. 72 shows one form. The copper cathode is weighed before and after the current flows. The weight of copper deposited and the time taken are used to calculate the current strength.

=76. Unit of Quantity=; =The Coulomb= is the quantity of electricity given, in _one second_, by a current having a strength of one ampere. Time is an important element in considering the work a current can do.

=77. Electrical Horse-power=; =The Watt= is the unit of electrical power. A current having the strength of one ampere, and an E. M. F. of one volt has a unit of power. 746 watts make one electrical horse-power. Watts = amperes × volts. Fig. 73 shows a direct reading wattmeter based on the international volt and ampere. They save taking simultaneous ammeter and voltmeter readings, which are otherwise necessary to get the product of volts and amperes, and are also used on alternating current measurements.

There are also forms of wattmeters, Fig. 74, in which the watts are read from dials like those on an ordinary gas-meter, the records being permanent.

Fig. 75 shows a voltmeter V, and ammeter A, so placed in the circuit that readings can be taken. D represents a dynamo. A is placed so that the whole current passes through it, while V is placed between the main wires to measure the difference in potential. The product of the two readings in volts and amperes gives the number of watts.

=78. Chemical Meters= also measure the quantity of current that is used; for example, one may be placed in the cellar to measure the quantity of current used to light the house.

Fig. 76 shows a chemical meter, a part of the current passing through a jar containing zinc plates and a solution of zinc sulphate. Metallic zinc is dissolved from one plate and deposited upon the other. The increase in weight shows the amount of chemical action which is proportional to the ampere hours. Knowing the relation between the quantity of current that can pass through the solution to that which can pass through the meter by another conductor, a calculation can be made which will give the current used. A lamp is so arranged that it automatically lights before the meter gets to the freezing-point; this warms it up to the proper temperature, at which point the light goes out again.