228 OUR PHYSICAL WORLD 



that, when the current to be measured is sent through the wire, 

 the core turns on its pivot, repelled by the magnetic poles. When 

 the current ceases to flow, the iron core is returned to its original 

 position by the action of a spring. A hand like that of a watch 

 is attached to the core over the pivot, so that its free end moves 

 over a graduated scale on the face of the ammeter. The greater 

 the amperage, the greater the deflection of this hand. 



The voltage of the current is measured by a similar instru- 

 ment, the voltmeter. In this meter, a part of the main current 

 is shunted off through a fine wire wound about the core. The 

 greater the voltage, the greater the current that flows in this 

 wire and the more the needle is deflected. 



Both instruments may be combined in one, the voltammeter. 

 In this instrument the needle is deflected in one direction for 

 measuring the amperage and in the opposite direction for measur- 

 ing the voltage. 



It will be seen later that electrical energy may easily be 

 transformed into mechanical energy by means of the motor, and 

 that mechanical energy may be transformed into electrical 

 energy by means of the dynamo. Electrical energy is turned 

 into heat by such devices as the electric flatiron, the hot-point 

 heater, etc. It is convenient, therefore, to have exact equivalents 

 of electrical energy in terms of mechanical energy and of heat. 



It is found that a current of one ampere working under 

 pressure of one volt will do work equivalent to 1/746 of one horse- 

 power. This unit is known as the watt. It is evident then that 

 volts multiplied by amperes divided by 746 equals horse-power. 

 Electric current is usually sold at so much per kilowatt-hour, the 

 unit being a thousand watts of electrical energy furnished every 

 hour. The instrument for measuring this consists of a small 

 motor that runs on the current and turns cogs that operate the 

 hands on the dials by which the meter is read (Fig. 96). 



The kilowatt-hour equals 3,600,000 joules of heat energy. 

 Or one may express the heat equivalent of electrical energy in 

 calories by stating that the number of small calories equals .24 



