494 TRANSACTIONS OF THE AMERTCAN INSTITUTE. 



The wire surrounding an electro-magnet is arranged spirally, but the wires 

 carry the electric force around a magnetic needle are necessarily parallel with 

 each other above and below it. To fix in the memory the manner in which 

 the poles of a magnetic needle are changed, let a person imagine his spine to 

 be the wire and the electric current to pass from the head toward the feet ; his 

 extended arms will represent a needle above the wire ; his right hand will 

 represent the north pole, and left hand the south pole. By a simple mnemon- 

 ical rule this position may be remembered. S will stand for the south pole 

 and the sinister or left hand. If the current pass from the feet to the head 

 the poles are reversed. 



The Chairman then directed attention to 



Ohm's Law. 



Prof. Ohm, after investigation, announced the following law : The inten- 

 sity of the Voltaic current is equal to the electro-motive force divided by the 

 resistance. If I represent the intensity, E the electro-motive force, and R 

 the resistance, the law is expressed by the following formula : 



R 



The resistance of a conductor depends on three properties : 1st. Its con- 

 ductibility, which is a constant for each conductor. 2d. Its section; and 3d. 

 Its length. 



The resistance is inversely, as the section, and directly, as the length of the 

 conducting wire. In the case of a metallic wire, the greater the conducting 

 power of the metal, and the greater its diameter, the less the resistance, but 

 the greater the length of the wire the greater the resistance. If c represent 

 the conductibility of the wire, s the area of its transverse section, and I its 



length : then R = — •. Substituting this value of R in the first equation, 



we have, when £ is a constant, 



- E _ EXcs 



T — ~r" ' 



cs 



In a Voltaic battery, composed of different elements, the intensity of the 

 current is equal to the sum of the electro-motive forces of all the elements 

 divided by the resistance. The elements of a battery are usually of the 

 same kind, and the resistances are two : 1st. That ofi'ered by the liquid con- 

 ductor, called the internal or essential resistance ; and 2d. That ofiei'ed by 

 the wire connecting the plates outside of the liquid, called the external or 

 non-essential resistance. 



Upon the conclusion of his remarks, the Chairman announced the selected 

 subject for discussion, " The Manufacture of I3ells " Adjourned. 



American Institute Polytechnic Association, ) 

 February 2'dd, 1865. j 



Prof. D. L, Tillman presiding. 



Fine Chinese Carving. 



During the miscellaneous busiuess. Dr. Ilich exhibited a string of olive 

 pits, on which were some very fiue specimens of Chinese carving. He said 



