Magnetisation in the Length of Metal Rods. 



113 



It is less easy to estimate the strength of his current, since he gives 

 no galvanometer readings, nor any details as to the electromotive 

 force and resistance of his battery. The resistance of his coil he 

 states to have been 0*75 ohm, and that of the rest of the circuit 

 (exclusive of the battery) about 0*25 ohm, making 1 ohm in all. He 

 used a battery of twenty-five Bunsen cells, and, in his own words, 

 "the above interpolar resistance showed that the maximum effect of 

 magnetisation would be given by connecting the twenty- five cells five 

 in couple and five in series/' This implies that the resistance of the 

 battery as thus arranged would be not far from 1 ohm, which, unless 

 the cells were very small, is surprisingly high. Taking the electro- 

 motive force of a Bunsen cell to be 1'9 volts, the electromotive force 

 of Mayer's battery would be 9*5 volts, and the current 9*5/2=4*75 

 amperes = 0*475 C.Gr.S. unit. The magnetising force would there- 

 fore be about 157*5x0*475 = 75 nearly. But 1 ohm is probably too 

 high an estimate for the resistance of the battery. Assuming the 

 resistance of the battery, leading wires, and connexions to be 

 0*5 ohm (which is the lowest reasonably probable estimate), the 

 current would be 7*6 amperes = 0*76 C.G-.S. unit, and the magnetising 

 force 157*5 x 0*76 = 118 units. In point of fact the force was pro- 

 bably something more than 75 and less than 118. 



In my own experiments both the magnetising coil and the rods of 

 metal were much shorter than those used by Joule and Mayer. The 

 length of the coil is 11*5 cm. ; it contains 876 turns of wire 1*22 mm. 

 in diameter, wound in twelve layers on a brass tube with boxwood 

 ends. The internal diameter of the tube is 1*5 cm., that of the coil is 

 1'9 cm., • and its external diameter is 5*2 cm. The mean length 

 of the diagonals of the cylindrical layers of wire is 12 cm., and the 

 field at the centre due to a current through the coil is approximately 



876 



477-^7) C=918 C when C is expressed in C.Gr.S. units, or 91*8C when 



C denotes the current in amperes. The strongest current which I 

 have hitherto used was 3*27 amperes, and the greatest magnetising 

 force was 3*27 x 91*8 = 300 units, Joule's maximum having been 126, 

 and Mayer's, on the most favourable estimate, not greater than 118. 



The apparatus for performing the experiment, which is of a very 

 simple nature, is shown in fig. 1.* A mahogany table, TT, supported 

 by three stout legs, the lower ends of which are let into a base board, 

 G, carries a lever arrangement for causing the elongating or retract- 

 ing rod under examination to deflect a mirror, M, which turns about 

 its horizontal diameter upon knife edges. The lower end of the rod 

 rests in a conical recess formed in a brass plate which is attached to a 

 hinged board : tlie height of the plate can be adjusted by turning a 

 screw, S. The rod passes through the coil, and also through a hole 



# The diagram is not drawn to scale. 

 VOL. XL. I 



