Electricity and Galvanism. 255 



nishes us with large quantities of electricity of tolerably high tension, 

 and possesses advantages for medical purposes which no other mode 

 of exciting electricity affords. To illustrate the mode of exciting 

 electricity by induction in the simplest manner, I will connect this 

 piece of copper wire wound into a circular coil with the terminal 

 screws of a galvanometer. 



N. S. The magnetic needle of the galvanometer, D. 

 E. The circular wire coil connected with the wires of the galvanometer. 

 A. B. The wooden cylinder covered with wire, the ends of which, Z. C. are 

 connected with the terminal plate of a battery. 



I have here a wooden cylinder, round which is wound a piece of in- 

 sulated wire, so as to form thirty or forty convolutions, and will place 

 this in the centre of the coil connected with the galvanometer. The 

 needle of the instrument is now at rest ; but observe what occurs the 

 instant I connect the ends of the wire coiled on the wooden cylinder 

 with the zinc and copper plates of a single galvanic battery. In an 

 instant the needle darts off, as if acted upon by some tangential force ; 

 and, after several violent oscillations through a considerable arc, it 

 slowly attains a state of rest, several degrees out of the magnetic 

 meridian. Now, as the wire on the cylinder had no connection what- 

 ever with that of the coil, it is obvious that the battery merely acted as 

 an exciting agent in disturbing the normal electric equilibrium of the 

 wire, causing the electricity to circulate in the form of current. This 

 current, you will observe, is of momentary duration, and is excited 

 only at the instant that the battery current first traverses the con- 

 ducting wire. But now, the needle being perfectly quiet, I will sudden- 

 ly break contact with the battery, and once more the needle rushes out 

 of the meridian line, and traverses a considerable arc, but in a direc- 

 tion opposed to that in which it travelled when connection was in the 



