CHAP. L] ELECTRICITY AND MAGNETISM. 11 



of electrification produced is, however, by no means equal 

 to the amount of the actual mechanical friction ; hence 

 it appears doubtful whether friction is truly the cause of 

 the electrification. Indeed, it is probable that the true 

 cause is the contact of dissimilar substances (see Art. 

 73), and that when on contact two particles have 

 assumed opposite electrical states, one having + the 

 other , it is necessary to draw them apart before their 

 respective electrifications can be observed. Electrical 

 machines are therefore machines for bringing dissimilar 

 substances into intimate contact, and then drawing apart 

 the particles that have touched one another and become 

 electrical. 



LESSON II. Electroscopes. 



11. Simple Electroscopes. An instrument for 

 detecting whether a body is electrified or not, and 

 whether the electricity upon it is positive or negative, is 

 termed an Electroscope. The feather which was 

 attracted or repelled, and the two pith balls which flew 

 apart, as we found in Lesson L, are in reality simple 

 electroscopes. There are, however, a number of pieces 

 of apparatus better adapted for this particular purpose, 

 some of which we will describe. 



12. Straw-Needle Electroscope. The earliest 

 electroscope was that devised by Dr. Gilbert, and shown 

 in Fig. 6, which consists of a stiff straw balanced lightly 



Fig. 6. 



upon a sharp point. A thin strip of brass or wood, or 

 even a goose quill, balanced upon a sewing needle, will 



