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HOT-WATER HEATING SYSTEMS (I-8)
The problem. It is a common observation that when water to be heated, the heat is always applied to the bottom of the vessel. Is there a reason other than convenience for this practice ? Certain types of hot-water heaters have a short pipe arranged in the fire box of the stove. How is it possible to heat a large quantity of water by this arrangement when the water tank is far removed from the furnace or stove ? In the hot-water heating system the furnace is placed in the basement. How is the water made to circulate to the various rooms? These and other similar questions can be answered if we know how water behaves when heated.
What to use. Ring stand, large ring, wire gauze, two burette clamps, flask (500 cc.) with two-hole stopper to fit, wide-mouth bottle (bottom removed) with two-hole stopper, four feet of glass tubing, six inches of rubber tubing, and black ink. What to do. 1. Assemble the
apparatus as shown in figure 8. No air
Fig. 8 bubbles should be allowed between the stopper and the liquid in the lower flask. Fill the bottle with clear water after the apparatus has been assembled.
2. Warm the water in the flask slowly and watch for results. If the glass tubes are filled with air bubbles the action may be delayed.
Questions. Does the outside of the flask change in appearance when the burner is lighted? Why is the bottom of the wide-mouth bottle removed ? Describe the changes produced by heating the colored water. Explain why these changes occur. Does this experiment show why a teakettle is heated at the bottom ? Summarize the essential facts shown by the experiment.
Suggestions for report. Make a cross-section diagram of the apparatus used. Indicate the circulation of water by arrows and brief notes.
Reference work. 'Show all the points of likeness and difference between the water heater (figure 16 of the text) and figure 8. Prepare a diagram of the heating system in your home. See figures 15 and 121 of the text for suggestions.
Optional problems. Can you show the convection currents in a beaker of water by means of sawdust? How does the cooling system of an automobile work?
TEMPERATURE JUDGMENT (I-9)
The problem. The oldest and most common way of estimating temperature is to note whether a thing "feels” cold or warm. That this may not be an accurate and satisfactory way of ing temperatures is suggested by the fact that persons often disagree regarding the temperature of an object. The fact that a rug feels warmer than a bare floor, although at the same temperature, suggests that the body is not a very satisfactory standard. The
words " temperature" and "heat” are often confused. Temperature is a condition of objects which can be measured by means of thermometers. On the other hand, a certain amount of heat must be furnished to an object to warm it from one temperature to another.
What to use. Thermometers, both centigrade and Fahrenheit, beaker of lukewarm water, ice, wide-mouth bottle filled with hot water, and wide-mouth bottle filled with cold water.
What to do. 1. Assemble the apparatus as shown in figure 9.
2. Put the right forefinger in the cold water and the left forefinger in the hot water. Let the fingers remain in the water a short time, then remove both and place them at once in the beaker of lukewarm water.
3. Next examine the thermometer. What is the highest temperature that can be recorded on each scale ? the lowest? How many degrees does the smallest division represent on each scale ?
4. Place the thermometer in the cold water. Read the temperature without removing it. Record the temperature in the table below. Record the temperatures of the warm water and hot water. Record the temperature of the room and that obtained by placing the thermometer under the tongue.
Questions. Describe the sensation produced on the two fingers by the lukewarm water. What is the lowest temperature that can be read on each scale? Why is there a bulb on the instrument? What metals and liquids behave as mercury when heated ? What mistakes may one make in reading a thermometer ?
Suggestions for report. Record the readings in this table :
Reference work. Read sections 13 and 14. Show how to change centigrade readings into Fahrenheit and vice versa.
GRAPHICAL COMPARISON OF THERMOMETER
The problem. Since our senses are unreliable in making exact estimates of the temperatures of objects, special instruments have been devised. Several kinds of thermometers are in use. The principal differences are with reference to the scales with which they are provided. In America the Fahrenheit scale is commonly used in the home. In all science work the centigrade scale is found most convenient. What is the relation between these scales ?
What to use. Rule, cross-section paper, readings from the previous experiment, and a sharp pencil.
What to do. 1. Prepare a graph to change readings from one scale to another as follows: On the cross-section paper (p. 28) place a small dot with a small circle around it at the intersection of the lines for 0° C. and 32° F. Place a similar dot at the intersection of the lines for 100° C. and 212° F.
2. Draw a fine straight line (using a ruler) across the page connecting these two points.
3. By means of this line find what temperatures correspond to 68° F. and 30° C. Check your results by the methods given at the top of the page of cross-section paper.
Questions. Which thermometer scale is most convenient ? Why? How could the graph be changed so that it would show fractions of a degree?
Reference work. Make a list of the different kinds of thermometers that
you have seen in use. Make a neat drawing of one of them. What can you find about the history of the Fahrenheit thermometer? the centigrade thermometer ? Optional problem. Can you make a model thermometer ?