208 



ELEMENTS OF GENERAL SCIENCE 



any desired strength of current. The regulating device known 

 as a rheostat or a resistance controller and the devices for 

 measuring currents are hi such constant use that several 

 million dollars' worth of them are sold annually. 



In power stations these instruments are made with large 

 proportions in order that the scale may be seen from a dis- 

 tance. In the automobile garage they may be found on the 



Reading=9121 Reading=997 



Reading 999,900 



FIG. 102. Different types of watt-hour meter dials 



To read the dials of the upper left-hand meter, begin with the right-hand pointer. 



The last figure passed by the pointer is 1 ; on the second dial, 2 ; on the third 



dial, 1; on the fourth dial, 9. Therefore the reading is 9121. Read the other meters 



the dials of which are shown 



slate or marble switchboard where the storage batteries are 

 charged. The so-called " alternating current " (the direction 

 of the flow of the current reverses from 50 to 120 times each 

 second) requires measuring instruments different from those 

 just described. 



220. How to read the watt-hour meter. Service wires from 

 the main lines (fig. 101) are brought into the home, shop, or 

 factory. Near the place of entrance the watt-hour meter 

 will be found. Although this instrument may appear to be 

 a rather complicated piece of apparatus, it is really a very 



