Determinations of Magnetic Susceptibility. 



409 



third columns contain the results obtained after the wire had been 

 subjected to "Ons and Offs," when the reversed current was circu- 

 lating through the coil, the former corresponding to the case " On" 

 and the latter to the case " Off." The first column in the rest, that 

 is, 14 to 17, is subject to the same explanation as the first column in 

 12 and 13 ; while the second column contains the result obtained in 

 the same way as the third column in 12 and 13. 



The next wire experimented on was also a pure soft iron wire, but 

 not so soft as the last one ; it is marked " Bright Wire "* in the tables. 

 Its gauge is about No. 20 B.W.G., and its breaking stress is about 

 20 kilogs. The piece experimented on was also a metre long; its 

 radius was "0450 of a centimetre, and therefore its sectional area 

 "006362 square centim. ; 12 kilogs. weight was employed for " Ons 

 and Offs." The wire elongated 6'2 per cent, of its length, so that 

 now its length was 106*2 centims., and remained so during all the 

 rest of the experiment ; the area of its cross-section being now '00599 

 square centim. The Table II refers to this wire. As regards the 

 first and second columns headed 1 under Deflection, exactly the same 

 remark applies to this table as to the last table. The first and second 

 columns under any of the headings give the results in the cases of 

 "On" and " Off" respectively; and the third and fourth columns give 

 the results (both in the case of " Off ") obtained, the former imme- 

 diately after reversing the current in the coil, and the latter after the 

 operation of " Ons and Offs " had been performed while the reversed 

 current was kept flowing through the coil. 



The Table III contains the results for the " Steel Pianoforte Wire," 

 which was of the same gauge as the " Dark Wire," and which is 

 largely used in Sir William Thomson's sounding machines. The 

 breaking weight of this wire is said to be roughly 100 kilogs. The 

 length of the piece of the wire experimented on was a metre ; its 

 radius was '03755 of a centimetre, and therefore the area of its cross- 

 section was '004452 square centim. A weight of 16 kilogs. was 

 always used for " Ons and Offs." No elongation of the wire was 

 observed. To both the first and second columns under all the head- 

 ings in the Table III precisely the same remarks apply as to those in 

 the preceding tables ; while the third column, should there be one, 

 gives the result for the coil alone. 



The last of the thin wires experimented on was a glass-hard-tempered 

 steel wire, the results of which are exhibited in the Table IV. The mode 

 of tempering which was adopted is perhaps worthy of a passing notice. 

 A convenient length was cut from the same hank as the preceding 



* Further particulars regarding the elasticity, &c., of this wire are found in 

 Mr. J. T. Bottomley's interesting paper on the " Effects of Long-continued Stress 

 on the Elasticity of Metals," " Proc. Koy. Soc," vol. xxix (1879), p. 221. 



