Electricity for Boys Part 4

[Ill.u.s.tration: _Fig. 40._ CROSS SECTION OF DETECTOR]

HOW TO PLACE THE DETECTOR.--If the detector is placed north and south, as shown by the two markings, N and S (Fig. 37), the magnet bar will point north and south, being affected by the earth's magnetism; but when a current of electricity flows through the coil (B), the magnet will be deflected to the right or to the left, so that the pointer (O) will then show the direction in which the current is flowing through the wire (R) which you are testing.

The next step of importance is to _measure_ the current, that is, to determine its strength or intensity, as well as the flow or quant.i.ty.

DIFFERENT WAYS OF MEASURING A CURRENT.--There are several ways to measure the properties of a current, which may be defined as follows:

1. THE SULPHURIC ACID VOLTAMETER.--By means of an electrolytic action, whereby the current decomposes an acidulated solution--that is, water which has in it a small amount of sulphuric acid--and then measuring the gas generated by the current.

2. THE COPPER VOLTAMETER.--By electro-chemical means, in which the current pa.s.ses through plates immersed in a solution of copper sulphate.

3. THE GALVANOSCOPE.--By having a coil of insulated wire, with a magnet suspended so as to turn freely within the coil, forming what is called a galvanoscope.

4. ELECTRO-MAGNETIC METHOD.--By using a pair of magnets and sending a current through the coils, and then measuring the pull on the armature.

5. THE POWER OR SPEED METHOD.--By using an electric fan, and noting the revolutions produced by the current.

6. THE CALORIMETER.--By using a coil of bare wire, immersed in paraffine oil, and then measuring the temperature by means of a thermometer.

[Ill.u.s.tration: _Fig. 41._ ACID VOLTAMETER]

[Ill.u.s.tration: _Fig. 42._ COPPER VOLTAMETER]

7. THE LIGHT METHOD.--Lastly, by means of an electric light, which shows, by its brightness, a greater or less current.

THE PREFERRED METHODS.--It has been found that the first and second methods are the only ones which will accurately register current strength, and these methods have this advantage--that the chemical effect produced is not dependent upon the size or shape of the apparatus or the plates used.

HOW TO MAKE A SULPHURIC ACID VOLTAMETER.--In Fig. 41 is shown a simple form of sulphuric acid voltameter, to ill.u.s.trate the first method. A is a jar, tightly closed by a cover (B). Within is a pair of platinum plates (C, C), each having a wire (D) through the cover. The cover has a vertical gla.s.s tube (E) through it, which extends down to the bottom of the jar, the electrolyte therein being a weak solution of sulphuric acid. When a current pa.s.ses through the wires (D), the solution is partially decomposed--that is, converted into gas, which pa.s.ses up into the vacant s.p.a.ce (F) above the liquid, and, as it cannot escape, it presses the liquid downwardly, and causes the latter to flow upwardly into the tube (E). It is then an easy matter, after the current is on for a certain time, to determine its strength by the height of the liquid in the tube.

HOW TO MAKE A COPPER VOLTAMETER.--The second, or copper voltameter, is shown in Fig. 42. The gla.s.s jar (A) contains a solution of copper sulphate, known in commerce as blue vitriol. A pair of copper plates (B, B') are placed in this solution, each being provided with a connecting wire (C). When a current pa.s.ses through the wires (C), one copper plate (B) is eaten away and deposited on the other plate (B'). It is then an easy matter to take out the plates and find out how much in weight B' has gained, or how much B has lost.

In this way, in comparing the strength of, say, two separate currents, one should have each current pa.s.s through the voltameter the same length of time as the other, so as to obtain comparative results.

It is not necessary, in the first and second methods, to consider the shapes, the sizes of the plates or the distances between them. In the first method the gas produced, within a given time, will be the same, and in the second method the amount deposited or eaten away will be the same under all conditions.

DISADVANTAGES OF THE GALVANOSCOPE.--With the third method (using the galvanoscope) it is necessary, in order to get a positively correct reading instrument, to follow an absolutely accurate plan in constructing each part, in every detail, and great care must be exercised, particularly in winding. It is necessary also to be very careful in selecting the sizes of wire used and in the number of turns made in the coils.

This is equally true of the fourth method, using the electro-magnet, because the magnetic pull is dependent upon the size of wire from which the coils are made and the number of turns of wire.

OBJECTIONS TO THE CALORIMETER.--The calorimeter, or sixth method, has the same objection. The galvanoscope and electro-magnet do not respond equally to all currents, and this is also true, even to a greater extent, with the calorimeter.

CHAPTER VI

VOLTS, AMPERES, OHMS AND WATTS

UNDERSTANDING TERMS.--We must now try to ascertain the meaning of some of the terms so frequently used in connection with electricity. If you intended to sell or measure produce or goods of any kind, it would be essential to know how many pints or quarts are contained in a gallon, or in a bushel, or how many inches there are in a yard, and you also ought to know just what the quant.i.ty term _bushel_ or the measurement _yard_ means.

INTENSITY AND QUANt.i.tY.--Electricity, while it has no weight, is capable of being measured by means of its intensity, or by its quant.i.ty. Light may be measured or tested by its brilliancy. If one light is of less intensity than another and both of them receive their impulses from the same source, there must be something which interferes with that light which shows the least brilliancy. Electricity can also be interfered with, and this interference is called _resistance_.

VOLTAGE.--Water may be made to flow with greater or less force, or velocity, through a pipe, the degree of same depending upon the height of the water which supplies the pipe. So with electricity. It may pa.s.s over a wire with greater or less force under one condition than another.

This force is called voltage. If we have a large pipe, a much greater quant.i.ty of water will flow through it than will pa.s.s through a small pipe, providing the pressure in each case is alike. This quant.i.ty in electricity is called _amperage_.

In the case of water, a column 1" 1", 28 inches in height, weighs 1 pound; so that if a pipe 1 inch square draws water from the bottom it flows with a pressure of 1 pound. If the pipe has a measurement of 2 square inches, double the quant.i.ty of water will flow therefrom, at the same pressure.

AMPERAGE.--If, on the other hand, we have a pipe 1 inch square, and there is a depth of 56 inches of water in the reservoir, we shall get as much water from the reservoir as though we had a pipe of 2 square inches drawing water from a reservoir which is 28 inches deep.

MEANING OF WATTS.--It is obvious, therefore, that if we multiply the height of the water in inches with the area of the pipe, we shall obtain a factor which will show how much water is flowing.

Here are two examples:

1. 28 inches = height of the water in the reservoir.

2 square inches = size of the pipe.

Multiply 28 2 = 56.

2. 56 = height of the water in the reservoir.

1 square inch = size of the pipe.

Multiply 56 1 = 56.

Thus the two problems are equal.

A KILOWATT.--Now, in electricity, remembering that the height of the water corresponds with _voltage_ in electricity, and the size of the pipe with _amperage_, if we multiply volts by amperes, or amperes by volts, we get a result which is indicated by the term _watts_. One thousand of these watts make a kilowatt, and the latter is the standard of measurement by which a dynamo or motor is judged or rated.

Thus, if we have 5 amperes and 110 volts, the result of multiplying them would be 550 watts, or 5 volts and 110 amperes would produce 550 watts.

A STANDARD OF MEASUREMENT.--But with all this we must have some standard. A bushel measure is of a certain size, and a foot has a definite length, so in electricity there is a recognized force and quant.i.ty which are determined as follows:

THE AMPERE STANDARD.--It is necessary, first, to determine what an ampere is. For this purpose a standard solution of nitrate of silver is used, and a current of electricity is pa.s.sed through this solution. In doing so the current deposits silver at the rate of 0.001118 grains per second for each ampere.

THE VOLTAGE STANDARD.--In order to determine the voltage we must know something of _resistance_. Different metals do not transmit a current with equal ease. The size of a conductor, also, is an important factor in the pa.s.sage of a current. A large conductor will transmit a current much better than a small conductor. We must therefore have a standard for the _ohm_, which is the measure of resistance.

THE OHM.--It is calculated in this way: There are several standards, but the one most generally employed is the _International Ohm_. To determine it, by this system, a column of pure mercury, 106.3 millimeters long and weighing 14.4521 grams, is used. This would make a square tube about 94 inches long, and a little over 1/25 of an inch in diameter. The resistance to a current flow in such a column would be equal to 1 ohm.

CALCULATING THE VOLTAGE.--In order to arrive at the voltage we must use a conductor, which, with a resistance of 1 ohm, will produce 1 ampere.

It must be remembered that the volt is the practical unit of electro-motive force.

While it would be difficult for the boy to conduct these experiments in the absence of suitable apparatus, still, it is well to understand thoroughly how and why these standards are made and used.

CHAPTER VII

PUSH b.u.t.tONS, SWITCHES, ANNUNCIATORS, BELLS AND LIKE APPARATUS