HOW ELECTRICITY IS MEASURED 



259 



EXPERIMENTS OR DEMONSTRATIONS WHICH WILL 

 HELP ANSWER THE PROBLEM QUESTIONS' 



Experiment 160. How can the current flow and the 

 electrical pressure of a dry cell be measured? 



A common battery tester like the one shown in Figure 

 413 is usually constructed in such a 

 way that both the current flow in am- 

 peres and the pressure in volts can 

 be measured with it. Secure such a 

 tester and three dry cells. Measure 

 the current flow of each cell. Meas- 

 ure the electrical pressure of each cell. 

 Now connect the three cells together 

 in series, that is, the positive pole of 

 one to the negative of the next, and so 

 on. Repeat the measurements. Next, 

 connect the three cells in parallel, that 

 is, all the positive poles together and 

 all the negative poles together. Again 

 measure the current and the pressure. 

 In your notebook tabulate results of 

 the measurement and complete the fol- 

 lowing statements. 

 Current flow is measured in __ and electrical pressure 



in A single dry cell will give . of pressure. 



Make diagrams of three cells in series and three cells in 

 parallel. Complete the following statement. Three cells in 

 scries gave volts, while in parallel they gave volts. 



Activity 161. How are electric meters read? 



Locate the electric meter in your home and make a care- 

 ful copy of the dials and the position of the hand on each. 

 Examine the dial face shown in Figure 414. Notice that 

 the first dial on the right is numbered in such a way that 

 the hand turns to the right, the second dial hand turns to 

 the left, the third to the right, and the last one to the left. 



The right dial, marked 10, records ten- kilowatt hours 

 when the hand has gone around once, each division on the 

 dial being one kilowatt hour. The next dial, marked 100, 

 records one hundred kilowatt hours, and when the hand of 



FIG. 413 



10.000 



1.000 



100 



FIG. 414 



the 10 dial has made one complete revolution, or 10 kilowatt 

 hours, the 100 dial hand will have moved down to 1. When 

 the 10 dial hand has made ten turns, the 100 dial hand will 

 have gone around just once, and the next or 1000 dial hand 

 will have moved down to 1. When the 1000 dial hand has 

 moved once around the dial, the 10,000 dial hand will point 

 to 1. 



10.000 



1,000 



too 



FIG. 415 



In reading the meter, read the 10 circle first, the 100 next, 

 the 1000 next, and last the 10,000. Study the dials in Fig- 

 ure 415. Notice that the 10 dial hand points to 4. The 100 

 dial points between 6 and 7. Read the last number which 

 the hand passed, or 6. Read the 1000 dial 5, as the hand 



FIG. 416 



points between 5 and 6 and the 5 was the last number 

 passed. The 10,000 dial reads 1. The complete reading then 

 is 1,564 kilowatt hours. Always read the 10 dial first and the 

 others in order, reading the last number passed by the 

 hand. 



' See workbook, p. 94. 



FIG. 417 



Read the dials in Figures 416 and 417 and record them in 

 your notebook. Also read the meter reading at your home. 



OTHER INVESTIGATIONS WHICH YOU CAN MAKE 



1. Make a reading of your electric meter in two successive 

 weeks and determine the amount of electrical energy used. 

 2.. Learn to read a gas meter and a water meter. 



READINGS WHICH WILL HELP ANSWER THE 

 PROBLEM QUESTIONS 



How are electric currents measured? Everyone 

 knows that water cur- 

 rents flow through 

 pipes. Electric currents 

 are similar except that 

 wires instead of hollow 

 pipes are used. Elec- 

 tricity will not flow un- 

 less it has a complete 

 pathway as shown in 

 Figure 418. The elec- 

 tric current would not 

 flow from the battery 

 through the switch and 

 through the bell unless the points A and B were con- 

 nected by a wire or other conductor. 



Pressure furnished by force pumps makes water 

 current flow through pipes, while pressure furnished 

 by batteries or generators forces electric currents 

 through the wires of a circuit. Rough pipes tend to 

 hold back the flow of water through them, and con- 

 ductors also offer resistance to the flow of electric 

 currents. As shown in Figure 419 and Figure 420 a 



FIG. 



