209. A gravity oiling system has an oil tank placed 10 ft. above the bearings to be lubricated. The tank is connected by small tubes to the various bearings. If the specific gravity of the oil is .88, what pressure will the oil have at the bearings?

210. In Fig. 81 we have a hoist operated by a hydraulic ram in the top of the crane post. The motion of the ram is multiplied by the system of pulleys shown in the figure. What size must the ram be that a load of 10,000 lb. can be lifted with a water pressure of 72 lb. per square inch, the efficiency of the whole apparatus being 70%?

CHAPTER XIX

HEAT

122. Nature of Heat. Some of the effects of heat are very useful in shop work and every mechanic should know something of the nature of heat and of the laws which govern its applications to shop work.

Heat is a form of energy; that is, it is capable of doing work. This we see amply illustrated in the steam engine and the gas engine, where heat is used in producing work. The steam engine uses heat which has been imparted to the steam in the boiler. Part of the heat of the steam is changed to work in the engine and the rest is rejected in the exhaust. Heat is not a substance as was formerly supposed-it cannot be weighed and cannot exist by itself.

It is always found in some substances. We generally get heat by burning some fuel such as coal, wood, gas, or oil. In burning, the fuel unites with oxygen, one of the constituents of air, and this process, called combustion, generates the heat. We cannot get heat by this process, therefore, without air. No fuel will burn without a supply of air, and as soon as we shut off the air from a fire, combustion stops and no more heat is generated. A fire may continue to give off heat for some time after the air is cut off, but this heat comes from the cooling of the hot fuel in the fire. Of the heat generated during combustion, some of it goes through the furnace walls to the surrounding air; some goes to heat up the bed of coals and any object that may be placed in the fire to be heated; but the greater part of the heat goes off in the gases that are formed by the union of the fuel with the air. It is to save this heat that we sometimes see steam boilers set up in connection with the furnaces of large forge shops.

There are other ways of generating heat besides that of combustion. One method, that is coming into considerable use and which is especially interesting to shop men because of the ease with which it can be controlled, is by the use of electricity. We now have electric annealing and hardening furnaces for use in

tool rooms, where a close regulation of the heat is very desirable. Then there are the electric furnaces by which aluminum and carborundum are produced. We also have electric welding as an example of the production of heat from electricity.

Another method of heat generation that is frequently encountered in shops, often where it is not desired, is the production of heat from work. We have seen how heat is turned into work, but here we have work returned into heat. One common case of this is in bearings, where heat is produced from the work that is spent in overcoming the friction. Another example is seen in the heating of a lathe tool when it is taking a heavy cut, or in the heating of the tool when it is being ground. In either event, the work spent in removing the metal goes into heat.

123. Temperatures.-Temperature is the indication of the height or intensity of the heat in a body. Lowering the temperature means a removal of heat from a body, and raising the temperature means the addition of more heat. The common method of measuring temperature is by means of an instrument known as a thermometer, which usually consists of a glass tube which is partly filled with mercury and which has the air exhausted from the other part of it. The mercury expands and contracts as the temperature rises or falls and, therefore, the height of the column of mercury is a measure of the temperature. Alcohol is often used instead of mercury for outdoor thermometers where the mercury might freeze.

There are two kinds of thermometer scales in common usethe Centigrade (abbreviated C.) and the Fahrenheit (abbreviated Fahr. or F.). On the Centigrade thermometer the space between the freezing-point of water and the boiling-point at atmospheric pressure is divided into 100 equal parts called Degrees (represented by °) the freezing-point being marked zero (0°) and the boilingpoint 100°. The balance of the scale is then divided into spaces of equal length below zero and above 100° in order that temperatures higher than 100 and lower than zero may be read.

On the Fahrenheit scale the freezing-point of water is marked 32° and the boiling-point 212°, so the space between is divided into 180° (212° - 32°=180°). This scale is also marked with divisions below 32° and above 212° in order to make the thermometer read through a wider range. The Fahrenheit thermometer is used more commonly in the United States than the Centigrade, which is used extensively in Europe. The Centigrade scale is,

however, used in this country for most scientific work and is becoming so common that it is desirable to understand the relations between the two scales. Fig. 82 shows the relation of the

two scales up to 212° F. or 100° C.

F

210

200

190

180

170

160

212 F = 100°C

-100°c

BOILING

90

80

Since the same interval of temperature is divided into 100 parts in the Centigrade scale, and 180 parts in the Fahrenheit scale, each Centigrade degree is 188, or Fahrenheit degrees. Similarly, one Fahrenheit degree is of a Centigrade degree. A change of 30° in tempera- 212210ture on the Centigrade scale would equal of 30, or 54° change on a Fahrenheit thermometer. Likewise, when the mercury moves 27° on a Fahrenheit thermometer, it would move only of 27=15° on a Centigrade scale. In changing a reading on one thermometer scale to the corresponding reading on the other, it is necessary to remember that the zero points are not the same. The Centigrade zero is at 32° F. In other words, the two zeros are 32 Fahrenheit degrees apart.

To change a reading on the Centigrade scale to the corresponding Fahrenheit reading: First multiply the degrees C. by. This gives an equivalent number of degrees on the F. scale. To this add 32, in order to have the reading from the F. zero.

To change a reading on the Fahrenheit scale to the corresponding Centigrade

70

150

140

130

120

110

100

90

reading: First subtract 32. This gives the number of F. degrees above freezing (which is the C. zero). Multiply the result by 5, thus obtaining the desired C. reading.

Examples:

1. Change 30° C. to the corresponding Fahrenheit reading.

9

30X54, the equivalent number of F. degrees.

54+32=86° F., the reading on a F. thermometer.

2. What would a Centigrade thermometer read when a Fahrenheit thermometer stood at 72°?

72-32-40, the number of F. degrees above freezing.

40×5=222° C., Answer.

These rules or relations are often expressed by the following formulas, in which C stands for a reading on the Centigrade thermometer and F for a reading on the Fahrenheit thermometer.

The parenthesis () when used as above means that the work indicated inside of it is to be done first and then the result multiplied by §. In the second formula, the C is first to be multiplied by and then the 32 is added to the product. It is always to be understood that multiplications and divisions are to be performed before additions and subtractions unless the reverse is indicated, as was done in the first of these formulas, by the use of the parenthesis ().

When it comes to measuring the temperatures in furnaces, as is often desirable in fine tool work, a thermometer is clearly out of the question. As the mercury thermometer is ordinarily made, it should not be used for temperatures above 500°, but, by filling the glass tube above the mercury with nitrogen gas under pressure, a thermometer can be made that may be read up to 900°. For higher temperatures, devices called Pyrometers are used. There are numerous kinds of pyrometers, but the one most used in the shops for furnace temperatures is what is called the Le Chatelier pyrometer. In this pyrometer, one end of a porcelain tube about in. in diameter and from 12 in. to 40 in. long is thrust into the furnace and held there or, if frequent readings are to be taken, it may be placed there permanently. Inside this tube are some wires of special composition that generate an electric current when they get hot. From the other end of the tube a couple of wires run to a small box containing a "galvanometer," that is, a device for indicating the strength of the electric current generated. This has a needle swinging over a dial and the dial is usually laid off in degrees so the temperature is read direct. Most of these pyrometers have centigrade graduations, but one should be sure which scale a pyrometer has before he uses it.