CH. xv. THE FIRST ELECTRIC MACHINE. 121 



two halves of an orange with the inside taken out. These 

 hemispheres fitted tightly together, so that no air could pass 

 in or out when they were shut. Outside he fastened rings 

 to hold by, so as to pull them apart, and at the end of one 

 hemisphere he fixed a tap which fitted on to his air-pump. 

 Now, as long as there was air inside the closed globe the 

 two halves came apart quite easily ; but when he had drawn 

 out the air with the air-pump and turned off the tap so as to 

 leave a vacuum inside, it required immense strength to drag 

 the globe into two parts. This showed that the atmosphere 

 was pressing heavily on every side of the globe, forcing the 

 two halves firmly together, and as there was no air inside to 

 resist this pressure, the person trying to separate them had 

 to force back, as it were, the whole weight of the atmosphere 

 to get them apart. As Guericke was burgomaster of 

 Magdeburg, this experiment has always been called ' the ex- 

 periment of the Magdeburg hemispheres.' 



The first Electrical Machine made by Guericke. 

 You will remember that Gilbert had shown in 1600 that 

 sulphur and many other bodies, when they are rubbed, will 

 attract light substances. Since his time very little notice 

 had been taken of this fact, till Guericke invented the first 

 rough electrical machine in 1672. He made a globe of 

 sulphur which turned in a wooden frame, and by pressing a 

 cloth against it with his hand as it went round he caused 

 the sulphur to become charged with electricity. His 

 apparatus was very rough, but it led to better ones being 

 made; and some years later, in 1740, a man named 

 Hawksbee substituted a glass globe for the sulphur and a 

 piece of silk for the cloth, and in this way electrical 

 machines were made much like those we now use. 



Guericke also discovered that bodies charged with the 

 same kind of electricity repel each other. If you hang a 



