PHYSICS OF NINETEENTH CENT. ELECTRICITY. 585 



blished himself in that business on his own account. He was a work- 

 man of extraordinary intelligence and perseverance, and though himself 

 not a professor of science, he gave to science in 1851 one of its most 

 valuable instruments of investigation, and the best embodiment of the 

 principle of voltaic induction discovered twenty years before by Fara- 

 day. He made the induction circuit with a far greater number of turns 

 than had before been used, using fine wire very carefully insulated by 

 gum-lac between the successive layers. The centre of the coils in 

 Ruhmkorff's machine is occupied by a bundle of iron wires. Fig. 297 

 represents a large Ruhmkorff's coil. The inner coil which conveys the 



FIG. 257. RUHMKORFF'S COIL. 



battery current is formed of rather thick copper wire; the outer coil is 

 of fine covered wire very carefully insulated. The length of wire in the 

 outer coil is much greater than that of the inner, amounting in the larger 

 coils to 15 or 20 miles. The apparatus on the left is for the purpose 

 of automatically interrupting and renewing the battery current with 

 rapidity. It is, in the instrument represented in the figure, worked by 

 a small separate battery, which sets the small electro -magnet into 

 activity ; and this acts upon an armature attached to a rocking lever, 

 from which two platinum points descend into two small glass cups 

 containing mercury covered with a stratum of alcohol. At each move- 

 ment the points dip into the mercury, and this establishes a communi- 

 cation by which the current from the powerful battery is conveyed to 

 the inner or inducing coil. This little apparatus, known as Foucanlfs 

 Interrupter, was devised for the purpose of avoiding certain inconve- 



