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HOW DOES A FLASHỮIGHT OPERATE? (XVII-1)

The problem. The electric flashlight has been called the modern candle. As a constant source of light it is rather expensive to operate. Nevertheless, the great convenience and safety of the device contributes to its popularity. Many thousands of small dry cells are used for flashlights every year. The manufacture of miniature lamps has been greatly increased by the development of the flashlight. How is this appliance constructed and how does it operate?

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What to use. Flashlight, magnifying glass, connecting wire (copper), hammer, and shears.

What to do. 1. Remove the battery of dry cells and the lamp from the case. Examine the switch to see how it closes and opens the circuit. Make a cross-section drawing showing the path of the current from one terminal of the battery through the switch, lamp, etc. to the other terminal.

2. Study the construction of the lamp with a magnifying glass. How does the current enter and leave the small filament ?

3. Use a hammer to remove the wax from the tops of the cells. How is the carbon of one cell connected to the zinc of another ? How many cells do you find ? Connect a wire to the carbon of the first cell (Fig. 34) and to the zinc of the third, and light the

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lamp. Make a diagram showing the direction of the current from cell 1 through the lamp and each cell back to cell 1.

4. Test the lamp with two cells, and with one cell.

5. Disconnect the three cells. Join all the carbons with one wire and all the zincs with another (Fig. 34). Light the lamp. Make a diagram showing the path of the current from cell 1 through the lamp and each cell back to cell 1.

6. Cut the zinc so as to show the inside of the cell, and compare the parts with figure 98, A, of the text.

Questions. What are the essential parts of an electric circuit? Is a fuse used here? Why? In what direction does the current flow in the circuit? in the cell ? Answer the questions in 2 and 3. How are cells connected in series ? in parallel ? Which is used here? Why?

Suggestions for report. File the corrected drawings in the notebook.

Reference work. Read sections 211 to 216.

Optional problems. Make a study of a different type of flashlight.

HOW IS THE COST OF ELECTRICITY

DETERMINED? (XVII-2)

The problem. As a source of heat and light electricity is more expensive than gas or kerosene, but there are compensations in the form of efficiency and cleanliness. Because of these advantages the use of the electric current in the house is increasing. In order to determine the cost of the energy used, the company usually installs a watt-hour meter, which is read at regular intervals by an agent of the power company. These readings are used as a basis of the bill which is sent to the consumer. How are these readings obtained ?

What to use. The electric meter of the home, school, or store; figures 101 and 102 of the text; and blank forms for meters.

What to do. 1. Study carefully sections 219 and 220 in the text. Learn how to read the watt-hour meter (Fig. 35) and how to compute the bill for the month.

2. Read the meter on the same date of each month and enter the positions of the pointers in the circles. Find the local cost per kilowatt and compute the cost per month. Record the number of lamps and other electric devices in your home.

Questions. Tell how to read the meter. What mistakes are easily made ? What is a kilowatt? a kilowatt hour ? Can you estimate the average energy used per hour for each lamp and compare this with the values given on page 212 of the text ? What are the sources of excessive light bills ?

Suggestions for report. Complete all calculations and enter the correct values for a final record.

Reference work. Read sections 221 to 225. Secure the monthly electric bills for one year from a home, school, or shop, and make a bar diagram showing the values for each month during the year.

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Optional problems. Construct a large mechanical model which can be used for demonstrating how to read a watt-hour meter. Make a list of the motors in the school building, giving the information found on the name plate. What is the total horse power ?

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