212 Our Surroundings 



(ampere meter) having a scale marked in amperes. Notice the 

 action of the ammeter on any automobile when its lights are 

 turned on or off. 



The Ohm. An electric current meets with resistance in 

 passing through any conductor. The length, size and material 

 of the conductor affect the amount of resistance. Consequently 

 the electrician must make allowance for resistance in planning 

 all electrical work. The unit for measuring resistance is the ohm. 

 One ohm equals the resistance offered by 157 feet of number 18 

 copper wire, such as is used in doorbell wiring. 



The Volt. To set anything in motion, force or pressure must 

 be applied. This is as true of electric currents as of solids. To 

 keep an electric current flowing between two points in a circuit 

 there must be a difference in potential, or amount of electrical 

 charge, at the two points to cause pressure. This pressure is called 

 electromotive force. It is measured in volts by an instrument 

 called a 'voltmeter. A volt is the electromotive force necessary 

 to cause a current of one ampere against a resistance of one ohm. 

 The ordinary voltage in the wiring of a house is 110 volts. 



The Watt. The amount of work an electric current can do 

 is measured by the watt. The number of amperes of current 

 multiplied by the number of volts gives the number of watts. 

 Electric light bulbs are marked in watts, to show the power of 

 the light produced. Bulbs of 40 watt capacity are most commonly 

 used in homes. 



Power for running machinery and all lighting plants is often 

 measured in kilowatt hours. A kilowatt hour represents the work 

 of 1000 watts for one hour. 



SUMMARY 



The exact nature of electricity is not known. Under suitable 

 conditions electricity will flow and exert pressure, or force. 



Static electricity is confined electricity produced by friction 

 or similar means. 



There are two kinds of electric charges, positive and negative. 



The electroscope is an instrument for finding out whether or 

 not an object is electrically charged. 



