Hawkins Electrical Guide, Number One Part 13

One _ampere-hour_ is simply another way of saying 3,600 coulombs. Of course 3,600 coulombs of electricity may be obtained in any desired time.

It all depends on the rate of flow or the current strength in amperes.

For instance, 2 amperes in 1/2 hour, or 4 amperes in 1/4 hour will also give one ampere-hour of 3,600 coulombs.

It is well to keep the distinction between coulombs and amperes in mind, as even in text books very lately published these units are confounded. To ill.u.s.trate further the difference between coulombs and amperes, the following example is given.

It is sometimes estimated that the quant.i.ty of electricity in a flash of lightning is 1/10 coulomb, and the duration of the discharge 1/20000 part of a second. What is the current in amperes?

Now since

coulombs = amperes seconds (1)

solving (1) for the current,

amperes = coulombs/seconds (2)

subst.i.tuting the given values in (2),

amperes= (1/10) / (1/20000) = 2000

=Power.=--The term power means _the rate at which work is done;_ it is usually expressed as _the number of foot pounds done in one minute_, that is

power = (foot pounds) / minutes

_Power exerted for a certain time produces work._

=Ques. What is the mechanical unit of power?=

Ans. The horse power.

=Ques. What is one horse power?=

Ans. 33,000 foot pounds per minute.

The unit is due to James Watt as being the power of a strong London draught horse to do work during a short interval and used by him to measure the power of his steam engines. One horse power = 33,000 ft. lbs. per minute = 550 ft. lbs. per sec. = 1,980,000 ft. lbs. per hour.

=Ques. What is one horse power hour?=

Ans. Work done at the rate of one horse power for one hour.

=Ques. What is the electrical unit of power?=

Ans. The watt.

=Ques. What is a watt?=

Ans. It is the power due to a current of one ampere flowing at a pressure of one volt. One watt = one ampere one volt. It is equal to one joule per second.

=Ques. What is a kilowatt?=

Ans. 1,000 watts.

=The Watt-Hour.=--The elements which may be measured are, however, not only the volume of current, the unit of which is the ampere, and time, the unit of which is the hour, but also the _pressure_, the unit of which is the volt.

It is evident that a perfect system of electrical measurement should take account of the total amount of energy consumed, and should depend not only upon the volume of current, but _also upon the pressure_ at which the current is applied.

The basis of such a system if provided in a unit which is the product of the two units of current and pressure, and which is termed a _volt-ampere_ or _watt_.

_The watt-hour represents the amount of work done by an electric current of one ampere strength flowing for one hour under a pressure of one volt._

EXAMPLE--An incandescent lamp taking one-half an ampere of current on a circuit having a pressure of 100 volts, or a lamp taking one ampere on a circuit having a pressure of 50 volts, would each be consuming 50 watts of energy, and this multiplied by the number of hours would give the total number of watt-hours for any definite time.

_The watt, then, is an accurate and complete unit of measurement and is generally applicable to all forms of electrical consumption._

[Ill.u.s.tration: FIG. 85.--Tyndall's experiment ill.u.s.trating the production of heat by friction. A bra.s.s tube about 7 inches in length and 3/4 of an inch in diameter, is fixed on a small wheel. By means of a cord pa.s.sing round a much larger wheel, this tube can be rotated with any desired velocity. The tube is three parts full of water, and is closed by a cork.

In making the experiment, the tube is pressed between a wooden clamp, while the wheel is rotated with some rapidity. The water rapidly becomes heated by the friction, and its temperature soon exceeds the boiling point. The pressure caused by the formation of steam forces out the cork and projects it to a height of several yards.]

A watt of electrical energy corresponds to 1/746 of a horse power of mechanical energy; hence, if a lamp or motor require energy equivalent to 1/746 of a horse power for one hour, it might be said to take one watt-hour.

=Mechanical Equivalent of Heat.=--The eminent English physicist, James Prescott Joule, worked for more than forty years in establis.h.i.+ng the relation between _heat_ and _mechanical work_; he stated the doctrine of the conservation of energy and discovered the law, known as Joule's law, for determining the relation between the heat, current pressure, and time in an electric circuit.

=Ques. What is heat?=

Ans. A form of energy.

Heat is produced in the agitation of the molecules of matter--the energy expended in agitating these molecules is transformed into heat.

[Ill.u.s.tration: FIG. 86.--Joules' experiment on the mechanical equivalent of heat, in which he caused paddle-wheels to rotate in a vessel of water by means of falling weights _W_. The amount of work done by gravity upon the weights in causing them to descend through any distance _d_ was equal to their weight _W_ times the distance. If the weights descended slowly and uniformly, this work was all expended in overcoming the resistance of the water to the motion of the paddle-wheels through it; that is, it was wasted in eddy currents in the water. Joule measured the rise in the temperature of the water and found that the mean of his three best trials gave 427 gram meters as the amount of work required to develop enough heat to raise a gram of water one degree. He then repeated the experiment, subst.i.tuting mercury for water, and obtained 425 gram meters as the work necessary to produce a calorie of heat. The difference between these numbers is less than was to have been expected from the unavoidable errors in the observations. He then devised an arrangement in which the heat was developed by the friction of iron on iron, and again obtained 425. This corresponds to 772 foot pounds, but later experiments show that the correct value is 778 foot pounds.]

=Ques. How is heat measured?=

Ans. In British thermal units (B.t.u.).

=Ques. What is the British thermal unit?=

Ans. The quant.i.ty of heat required to raise the temperature of 1 lb. of pure water 1 Fahr., at or near 39.1 F., the temperature of maximum density.

=Ques. What is the mechanical equivalent of heat?=

Ans. The number of foot pounds of mechanical energy equivalent to one British thermal unit.

Joule's experiments 1843-50 gave the figure 772 ft. lbs. which is known as Joule's equivalent. Later experiments gave higher figures, and _the present accepted value is 778 ft. lbs._, that is: 1 B.t.u. = 778 ft. lbs.

=Electrical Horse Power.=--It is desirable to establish the relation between _watts_ and _foot pounds_ in order to determine the _capacity_ of an electric generator or motor in terms of _horse power._

One watt is equivalent to one joule per second or 60 joules per minute.

One joule in turn, is equivalent to .7374 ft. lbs., hence 60 joules equal: