ELECTRICITY 161 



Experiment 85. Suspend a pith ball from the ring of a ring stand 

 by a very fine piece of copper wire no larger than a thread. Wrap 

 the wire around the pith ball in several directions. Bring a rubbed 

 glass rod toward the pith ball. Does it act as it did when suspended 

 by silk ? Allow the ball to touch the rod. Does the ball now act as 

 it did when suspended by silk ? Try these same experiments, using 

 the vulcanite rod. 



It was known by the ancient Greeks that when certain 

 substances, one of which was amber, were rubbed, they 

 had the power of attracting light objects. This property 

 was afterward called electricity, from the Greek word for 

 amber. From the previous experiments it has been seen 

 that when glass is rubbed with silk, and vulcanite with 

 flannel or a cat's skin, they seem to have two different 

 kinds of electrical charges. The like kinds repel each 

 other and the opposite kinds attract. These two kinds 

 are called positive and negative respectively. 



Whether there are really two kinds of electricity has not 

 yet been fully determined, but electricity acts exactly as it 

 would if there were two kinds, and it has become customary 

 to speak as if there were. In Experiment 84 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 85 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, nonconductors. 



Experiment 86. Having started the electrical action in a static 

 electrical machine (Fig. 75), 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 effect 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 



