the Laws of Magnetic Forces. 289 



20. The truth of this result was in a great measure confirmed, 

 by ascertaining the absolute weight required to overcome the 

 attractive force induced in a mass of iron at different distances 

 from a magnet. The requisite apparatus for such an experi- 

 ment is very easy to be constructed. There is a vertical support 

 of wood cd, Fig. 10, sustained at a convenient height on two or 

 three columns ef, by means of a horizontal plane d, the columns 

 ef being screwed into a circular base *, of a convenient size. 

 The magnet AB, and iron a b, to be submitted to experiment, 

 are secured in the required position by the moveable bands 

 of brass before described (11), the iron passing below through 

 the plane at d. There is a portion, a B, of the vertical support 

 c d, divided into inches and tenths of an inch, to mark the re- 

 lative distances by which the iron and magnet are separated. 

 A ring of soft steel r, about an inch and a half in diameter, ha- 

 ving a light brass pan S attached to it, is suspended from the 

 point r by the attractive force induced in the iron a b ; a slender 

 rod of brass passes through this ring r, being supported at each 

 extremity in the columns ef, in order to prevent the ring from 

 falling an unnecessary distance when the force of the attraction 

 is overcome by weights placed in the pan at S *. 



21. A magnetic bar being selected, two feet in length, an inch 

 and a half wide, and half an inch thick, it was placed, by means 

 of this contrivance, at different distances from a mass of iron of 

 the same breadth and thickness, but not exceeding three inches 

 in length. When the magnet and iron were two inches apart, it 

 required, as determined by various trials, between 190 and 210 



* The point r consists of a very short piece of soft iron, about two-tenths of an 

 inch in diameter. It is screwed firmly into the centre of the iron ab, so as to have 

 a perfect contact, and projects vertically for about the one-tenth of an inch from its. 

 lower extremity ; thus the steel ring r becomes always attached in the same place.. 



