HOW ELECTRICITY IS CONTROLLED 



pressure of about 1.5 volts. A volt is the unit used in 

 measuring electric pressure, or the force which causes 

 electricity to flow through a conductor. An instrument for 

 measuring voltage is called a voltmeter. 



Electric current. If we use our tank again for com- 

 parison, we shall see that water pressure alone will not tell 

 us how great a flow of current may be obtained. The 

 size and length of the pipe help to determine this, be- 

 cause they regulate the resistance, and resistance is one 

 factor which determines the amount of water which flows. 



Equal 

 Pressure 



Pipe 1/argfe. T>ia . ^ \Pipe 5maU Dice 

 (Low re5istance.)|P^\ (High resi?tct 

 Heccv-y "Flow, 



Tlow. 



Wire LargeDiameter Wire Simll Dia.. 

 (jow resistccixcje.) OughresistctJxcO 

 L-arcffe Guerrejxt, Small Current 



Water analogy : relation of current to resistance in large and small wires. 



This also holds in a similar manner with electricity. If 

 we have the poles of a cell connected by a wire of small 

 resistance, a large current will flow through it, but if the 

 resistance be great, a small current will flow. The unit 

 for measuring an electric current is the ampere, so named 

 after a French scientist. The instrument by which this 

 current is measured is the ammeter. 



How to join cells for use in the home. There are two 

 ways in which we may join cells together so that we can 

 overcome resistance offered by wires. These are in series, 

 and in parallel. We can understand these terms best by re- 

 ferring again to our water tanks. Suppose we have three 



