1886.] Electromagnets and the Magnetisation of Iron. 489 



I proceed to describe the experiments in greater detail. 



For several reasons it appeared desirable that the apparatus should 

 be on as small a scale as conveniently possible. Accordingly, I chose 

 for the rods a piece of iron wire, the diameter of which as measured 

 by a micrometer gauge was 2 '64 mm. The length of each rod was 

 12 cm. when straight, and 10*5 cm. when one end had been bent into 

 the form of a hook. The flat ends were rendered as true as was 

 possible by means of a lathe with a slide rest, but they were not 

 ground. The bobbin consisted of a brass tube 4 mm. in internal 

 diameter and 112 mm. in length fitted with ebonite ends. Around 

 this insulated copper wire, 0'7 mm. in diameter, was wound in nine 

 layers, each layer containing 104 turns. The internal diameter of the 

 coil of wire was 6 mm., its external diamater 21 mm., and its length 

 between the ebonite ends 99 mm. The magnetic field at its centre 

 produced by a current of C amperes was therefore 



^ X 9-9 104 cos 7 ° 41 ' x 10 c = 118 c vei T nearl y- 



The coil was fixed in a vertical position beneath a small table. One 

 of the iron rods was passed into the coil through a hole in the table 

 and suspended by its hooked end from a horizontal brass bar, the 

 height of which was so adjusted that the flat end of the iron was 

 exactly in the centre of the coil. A scale-pan for carrying the 

 weights was attached to the hook of the other iron rod. Around 

 each rod near the flat end two or three layers of paper were gummed, 

 of sufficient thickness to serve as a guide and ensure that the two 

 ends should meet concentrically inside the coil, but not so thick as to 

 cause any material friction. 



The source of electricity was a battery of 27 Grove cells, and the 

 strength of the current was varied by varying the number of cells in 

 use, or when there was only one cell by inserting resistance. The 

 current was measured by a tangent galvanometer of Helmholtz's 

 pattern (by Elliott), having two rings of thick copper wire. With 

 the stronger currents the heating effect was found to be so great that 

 the circuit could not be kept closed for more than a fraction of a 

 second. This of course did not allow sufficient time to make a 

 galvanometer reading. A spiral of bare German silver wire was 

 therefore provided of the same resistance as the coil when cold, 

 through which the current could be diverted by means of a switch, 

 and the galvanometer reading leisurely made. 



The method of proceeding was as follows : — A certain number of 

 battery cells having been arranged so that they could be thrown into 



(see Table II, cols. 1 and 2), it appears that the lifting power due to the magnetisa- 

 tion of the iron only was 14,993 grams per square centimetre, or 2133 lbs. per 

 square inch. — [July 1, 1886.] 



