

1903.J Expansion of some of the less Magnetic Metals. 371 



just as was shown by Nagaoka* for iron and nickel. But step by 

 step these inferences have been proved to be fallacious, as greater 

 care was taken in the exclusion of iron from the apparatus and in 

 the exact setting of the rod symmetrical in the coil and free from 

 mechanical connection with it. The final conclusion reached is that no 

 true expansion, positive or negative, can be detected within the limits 

 of the experiment. 



2. The magnetic and length-measuring apparatus have been de- 

 scribed in previous communications f and cannot easily be summarised 

 intelligibly. 



I am indebted to Mr. Schott (see the Note appended) for working out 

 the value of the reduction factor K in the expression 



H = 4;rNy . K. 



The ordinary expression H = 4irNy requires modifying for such a 

 short solenoid as the one used by me. The numerical calculation of 

 the integrals in the note gives for K the value 0'867. 



The rods are cylinders, 19 cm. long, 6 mm. diameter (except bismuth 

 and lead 12 mm. diameter). 



The coil is 19 cm. long and has 3604 turns, hence N = 190. 



Using the values of currents (7) below, we derive the corresponding- 

 value of field (H), where H is the effective uniform field as expressed 

 above. 



7 (in amperes)... 0-80 1-45 2'00 2'50 3-10 3'80 4*43 5'30 6-40 



7-70 9-25. ? 



H (c.g.s. units) .. 164 295 413 510 635 784 900 1090 1313 



1588 1906. 



These currents were read on an ammeter which was periodically 

 calibrated. They were always applied gradually by using a sliding 

 liquid resistance, vanishing to nil. 



The coil was made somewhat short. This was done in order, with a 

 given resistance in the windings, to concentrate the field on the rod. 



Again, the rod occupied the whole length of the coil. It is 

 customary when experimenting on a ferro-magnetic to build the core 

 up of three parts, the ferro-magnetic occupying the central third of the 

 length (where the field is practically uniform), whilst the two end 

 thirds of the length are of brass. Here the assumption is made that 

 brass itself undergoes no expansion. I could not proceed on this plan, 

 for we have no certainty that any metal is neutral, i.e., is quite free 

 from magnetic expansion. So I decided to have the rod the same 

 length as the coil and trust to calculation to obtain the mean effective 



* Loc. eit. 



t Shaw and law?, ' Electrician,' Feb., 1901, and Feb., 1902. 



2 D 2 



