312 



Mr. J. P. Joule^s Account of EarperimmU 



The bar electro-magnets were suspended in succession, vertically, 

 from the beam of a balance ; the corresponding wire electro-mag- 

 nets being brought vertically underneath, so that -j^th of an inch 

 intervened between the poles of the two magnets. Electrical cur- 

 rents of the quantities exhibited in the table were passed through 

 the continuous line of conductors presented by the galvanometer 

 and electro-magnetic coils. ITie attraction was measured in 

 grains by ascertaining the weight required to raise the suspended 

 electro-magnet from a piece of wood ^th of an inch thick, placed 

 on the upper pole of the lower electro-magnet. 



In order to vary the above experiments, and with a view to 

 ascertain what effect an increase of length would produce, I con- 

 structed ten more electro-magnets of the same sectional areas, 

 but 14 inches long, or double the former length, and wound with 

 twenty-two yards, or three times the length of similar insulated 

 wire to that previously used. Nos. VI. and VII. were made of 

 square ii-on wii'e ; the rest of bar iron. 



j.rtoii«L«i. Table 11. 



Eveiy one of the magnets used in the above series of experi- 

 ments, except Nos. VI. and VII., was wound to two thick- 

 nesses by the wire ; and in the large ones the iron was left un- 

 covered at equal intervals. I must mention, however, that 

 Nos. VI. and VII. had to be wound to three thicknesses in some 

 parts on account of their small size. On this account the mag- 

 netic attractions of these two pairs were somewhat deteriorated. 



It does not appear from the experiments that any great loss 

 of power arises from an increase of the length of the mag- 

 nets. It is plain, that, as the magnets in Table II. are wound 

 with three times the length of wire, twenty-four of current elec- 



