592 



HISTORY OF SCIENCE. 



a screen is reflected from the little mirror, and forms an image on the 

 graduated scale c. Thus, the minute delicately-suspended magnet is 

 provided in the beam of light with a weightless index-finger several 

 feet in length, and the instrument will indicate currents which would 

 altogether escape less refined methods of detection. 



Besides the great discoveries which have been already treated of, 

 numbers of others of much interest have been made, and many new 



FIG. 303. HOLTZ MACHINE. 



forms of apparatus have been constructed, not only for the newer 

 branches of electrical science, but in connection with that form of elec- 

 tricity which was the only one known up to the end of last century. 

 Some of the newer forms of apparatus would appear very amazing to 

 the older electricians. We have space to refer only to two of these, 

 which at the time they were first constructed made some sensation in 

 the scientific world. 



In 1840 Mr. (now Sir) VV. ARMSTRONG, of Newcastle-on-Tyne, found 

 that under certain circumstances steam arising from a boiler under 

 high pressure was charged with electricity. After some experiments 



