I >i: MKNSIONS OF RINGS AND RODS OF IRON AND OTHER METALS. 223 



The tables contain twenty columns, which, for convenience of reference, are lettered 

 from A to T. 



A gives the number of GROVE'S cells used. 



B the resistance in ohms inserted to vary the current when only one cell was in use. 



C shows the scale divisions indicated immediately before the circuit was closed (i.e., 



the temporary zero), the metal having been recently demagnetised. 

 D gives the readings obtained on closing the circuit for the first time after 



demagnetisation. 

 E contains the differences of C and D, giving the elongations or retractions in scale 



divisions. 

 F gives the scale readings immediately before the circuit was closed the second 



time (another temporary zero). Owing to small changes of temperature, the 



position of the wire was nearly always different by several divisions from that 



denoted in column C. 

 G gives the deflections when the circuit was closed for the second time after 



demagnetisation. 



H contains the differences of F and G. 



I, J, K are repetitions of C, D, E, the rod having been again demagnetised. 

 L, M, N are repetitions of F, G, H. 

 O gives the means of E and K. 

 P the means of H and M. 



Q contains the ammeter readings of the currents. 

 It the currents in amperes obtained by multiplying the ammeter readings by the 



factor 0'1356 or I '356, according as the coils of the instrument were arranged 



(by the commutator) in series or in parallel. 

 S gives the field at the middle of the coil, obtained by multiplying the amperes 



by 92. 

 T shows the elongations and retractions in ten-millionths of length obtained by 



multiplying the figures in column O by 1 "8 for the iron and by 27 for the 



cobalt. 



The first of the experiments with the iron rod was made on Dec. 19, 1887, the 

 second on Jan. 4, 1888, the whole apparatus having been in the meantime dismantled 

 and the rod removed from the coil, which had been used for another purpose. 



In the first experiment currents of descending as well as of ascending strength 

 were used. In the second only one smaller current was applied after the strongest, 

 the experiment being then stopped. 



The following points should be noticed : 



(I.) The very close agreement between the results obtained on different days, 



