HEAT AND LIGHT FROM ELECTRIC CURRENTS 199 



are also protected by fuses (fig. 97, #). All electric circuits 

 and the devices and appliances in them are protected either 

 by fuses or by automatic switches which open before the 

 current reaches a strength that will do damage. Insurance 

 companies do not take risks upon buildings that are improp- 

 erly wired. For experimental work it is wise to use a fuse 

 rated at a much lower value than that which the instrument 

 or device can carry safely, since certain fuses often carry a 

 momentary current from 50 to 100 per cent greater than the 

 rated value marked on the label. 



When the currents become very large, as those required 

 for street-car motors or commercial machinery, a device called 

 a "circuit breaker" is used. This is essentially an automatic 

 switch which opens, often with considerable noise, when the 

 current reaches a certain value. 



213. How electric circuits are opened and closed. We often 

 push an electric button or turn a switch of an electric-light 

 bulb without knowing what we are really doing. These are 

 forms of electric switches. The essential thing in all electric- 

 switches is that they complete the electric path by bringing 

 two .conducting metal surfaces together. This is shown by 

 means of the knife switch in figure 9>. If we place such sub- 

 stances as a silver dime, a copper penny, or a piece of brass 

 firmly between the blade of the switch and the metallic jaws, 

 the current flows through the circuit. Such substances are 

 called good conductors of electricity. Other materials, such 

 as glass, porcelain, mica, cloth, or rubber, which do not permit 

 the current to flow, are called nonconductors or insulators. 

 Where have you seen insulators used ? 



In speaking of an electric circuit we often say that it 

 is an open circuit, a closed circuit, or a short circuit. An 

 open circuit is one in which the metallic wires or other con- 

 ductors are broken at one or more points so that there is 

 not a complete path for the electricity. A closed circuit 

 is obtained when the switch blade is brought into contact 



