the Magnetic Behaviour of fixed Iron Filings. 135 



While experimenting with these figures, it occurred to the 

 author to try the effect of leading the current of a powerful 

 battery into the plate and of writing on it with the other pole. 

 This done, fine iron-filings were dusted over the plate ; and on 

 gently tapping it the writing became legible immediately. A 

 small thin disk of steel which thus had a current passed through 

 its centre exhibited afterwards a magnetism distributed in 

 small concentric rings. 



III. On Magnetic Figures for Demonstration. 



For the production of magnetic figures filings of wrought 

 iron are usually employed, though cast iron answers fairly. 

 Finely powdered magnetic oxide is recommended by some 

 writers, though it does not appear that its employment is 

 attended with any great advantage. Professor A. M. Mayer 

 took special pains* to produce filings of even quality from 

 specially prepared Norwegian iron; but he says nothing about 

 the size of filings he found best suited for the purpose. Fa- 

 raday made the remark f that " large and also fine filings are 

 equally useful in turn." 



The author, desiring to obtain figures on a larger scale than 

 usual, for purposes of class demonstration, used a number of 

 small steel needles with success. In the case of thin elongated 

 bodies such as these, the magnetic moment is great as com- 

 pared with the mass; hence it was to be expected that filaments 

 of fine soft iron wire would also yield a good result. Accord- 

 ingly he had a quantity of fine iron wire gauze of 32 meshes 

 to the inch cut to fragments. The filaments thus produced 

 were scattered in the usual way by means of a pepper-box with 

 perforated lid. The figures given by these filaments with 

 large magnets possess very well-marked characters, and are 

 decidedly superior to those made with ordinary filings. 



IV. On the Magnetic Behaviour of fixed Iron Filings. 

 Having occasion to draw the attention of his students to 

 the property of the lines of magnetic force as being at every 

 point tangential to the position of a small freely-suspended 

 magnet placed above them, the author, placing thus a small 

 magnet over the filings fixed some time previously to glass, 

 and from which the magnet producing them had been removed, 

 noticed k that they still retained their magnetic property. It 

 then occurred to him to see whether they still possessed direc- 

 tive force as a whole, and found that they appeared capable of 

 attracting and repelling a lightly suspended needle. A figure 



* Vide American Journal of Science, 1872. 

 t ' Experimental Researches/ vol. iii. p. 398. 

 L2 



