482 MAGNETISM AND ELECTRICITY 



to speak as if there were. In Experiment 154 it was found 

 that pith balls suspended by a silk thread could be charged 

 with electricity if brought in contact with a charged body. 

 Experiment 155 showed that this was not possible when 

 they were suspended by a copper wire. The wire conducted 

 the electricity away. Substances like copper that conduct 

 electricity are called conductors, and those substances like 

 silk which will not conduct it, non-conductors. 



A FLASH OF LIGHTNING 



Experiment 156. Having started the electrical action in a static 

 electrical machine (Figure 153), pull the knobs as far apart as the 

 spark will jump and notice the course taken by the spark. Does it 

 travel in a straight line? Hold a piece of cardboard between the 

 knobs so that its edge is just within the line joining them. What ef- 

 fect does the cardboard have upon the direction taken by the spark ? 

 Place the cardboard so that it entirely covers one of the knobs. 

 Is the spark able to pass through the card ? Attach a wire with a 

 sharp point to each of the knobs and extend it vertically two or 

 three inches above the knob. Start the machine. Do sparks 



