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XLIX. Mean Intensity of Magnetization of Soft Iron Bars of 

 Various Lengths in a Uniform Magnetic Field. By A. 

 Tanakadate*. 



[Plates IV. & V.] 



THE series of experiments now to be described on the 

 mean intensity of magnetization in soft iron bars of 

 various lengths has been carried out in the Physical Labora- 

 tory of Glasgow University, and in accordance with Sir 

 William Thomson's instructions. 



The experiments consisted in subjecting the piece of iron to 

 different strengths of magnetizing field, and in measuring 

 the magnetic moment thereby acquired. The amount of 

 residual magnetism was also observed by reducing the field 

 to zero after each magnetization. 



The magnetizing field was obtained as usual by means of 

 an electromagnetic solenoid consisting of fine insulated copper 

 wire, wound upon a brass tube. The length of the wound 

 part of the tube was 11*9 centim., and the total number of 

 turns 13158, so that the magnetic field due to 1 ampere was 

 4t7 13158/11-9x10 = 1390 in C.G.S. units, neglecting the 

 effect due to the broken ends. The current through the 

 solenoid was varied by inserting resistances by steps. Its 

 strength was measured by observing the difference of poten- 

 tials at the terminals of the solenoid by means of one of Sir 

 William Thomson's graded potential galvanometers f. 



The solenoid was laid at right angles to the magnetic 

 meridian, and in the prolongation of the axis of the solenoid 

 was placed a reflecting mirror-magnetometer. Under these 

 circumstances, the deflections of the magnetometer consist of 

 two parts, one due to the solenoid and the other due to the 

 magnetized iron in it. The first of these were taken account 

 of by making a separate observation on the magnetometer 

 deflection without the iron in it, before each experiment. 



The iron pieces were cut the required length, and were 

 heated to redness in a gas flame and slowly cooled : the 

 oxide coating formed on the surface was removed by rubbing 



* Communicated by Sir William Thomson, having "been read before 

 the British Association at the Bath Meeting-, 1888. 



t The resistance of the solenoid was 406*5 ohms, at 50° F., and was 

 assumed to be constant throughout the experiments ; as the temperature 

 of the room was never far from that temperature, and the strength of 

 current was never greater than T V ampere, the error arising from the 

 variation of resistance due to that of temperature could not have been 

 anything serious. 



