210 Tints. A. day und K. T. J-mes. 



Measurement <>f 



The magnetisation of the iron was determined in separate experi- 

 ments. For this purpose a narrow glass tube, 60 cm. long, was filled 

 with a number of lengths of iron wire, cut from the same specimen, 

 from which the insulation had been removed. A number of turns of 

 fine insulated copper wire was wound on the glass tube near its middle 

 and connected to a ballistic galvanometer, standardised by solenoid 

 and secondary coil. The tube, with the lengths of iron wire, was 

 placed within the magnetising coil, and the BH curve of ascending 

 reversals was determined for the iron in the usual way, and the mag- 

 netisation I calculated from the equation I = B - H/4jr. No account 

 was taken of the demagnetising factor of the iron wires ; the factor for 

 a cylinder of the same length and of cross-section equal to the sum of 

 the section of the thirteen pieces of wire in the glass tube is but of the 

 order 0-0008. 



Results. 



The following are particulars as to the coils, &c., used : 



Diameter of soft iron wire used in. resistance and 



magnetisation experiments ........................ 0'0745 cm. 



Resistance of iron coil P (longitudinally wound) ... 1 -045 ohms. 

 Resistance of iron coil Q (spirally wound) ......... 1 1 1 8 



Do., do., auxiliary coil A .............................. .1 -382 



Do., do., auxiliary coil B .............................. 1 -476 



The coils A, B were of German silver. Resistance per centimetre of 

 bridge wire (a-) = 0*0000536 ohm. The difference of the fractional 

 increments of resistance of P and Q was calculated from the approxi- 

 mate equation 



AP AQ A + B 

 **-T T AB-"*' 



where 8x is the displacement of the contact S (fig. 1) from its zero 

 position, i.e., from its position when the wires were not magnetised. 



The mean temperature of the iron wire during both the change of 

 resistance and the magnetisation measurements, determined by fre- 

 quently observing the temperature of the water entering and leaving 

 the magnetising coil, was 5'5 C. 



The results of all the measurements are shown in the accompanying 

 curves. Fig. 2 is the magnetisation curve (I, H, c.g.s.) of ascending 

 reversals. Fig. 3 the curve B, H for the longitudinal coil. Fig. 4 

 shows A</> as a function of the magnetising field H. The general resem- 

 blance between this curve and the magnetisation curve suggests that 

 the change of resistance depends on the magnetisation rather than on 

 magnetising force. 



