the Laws of Magnetic Forces. 285 



might say the magnetic bar at an inch distance exerted on our 

 suspended iron ce a force of 5 grains, supposing 5 1 grain ; and 

 thus the indications of such a machine, like the thermometer, 

 become universal *. 



14. Experimental inquiries concerning the laws of magne- 

 tic forces being, as already observed (2.), much embarrassed 

 by the complicated action which such forces exhibit, we are 

 first led to examine the absolute attractive force exerted be- 

 tween a magnet and a mass of iron, when placed at various dis- 

 tances from each other, in which case, we may consider that a 

 permanent magnetic developement exists only in one substance ; 

 but in this inquiry, it is essential to understand clearly the laws 

 and operation of induced magnetism, that is to say, the influence 

 which magnetised steel exerts upon ferruginous bodies not mag- 

 tic, so as to induce in them a developement of magnetic proper- 

 ties, such effect being the most simple case of magnetic action. 



15. For this purpose, the cylindrical piece of soft iron (13. 

 Note) was suspended from the wheel of the instrument, Fig. 1. : 

 it weighed 123 grains. The cylindrical counterpoise W being 

 about three-tenths of an inch diameter, which, by experiment, 

 gave 5 of attraction, equal to one grain. A mass of soft iron, 

 be, Fig. 5., two inches in length, eight-tenths of an inch wide, 

 and three-tenths of an inch thick, was then affixed by means of a 

 brass band n to the divided scale ; and immediately under this 



* There should be several small cylinders prepared of very soft iron, for general 

 use, being about two inches in length, and one quarter of an inch in diameter. The 

 iron-wire of commerce is convenient for the purpose. They should be accurately 

 turned, and great care should be observed in freeing them from any permanent po- 

 larity, which is readily done by making them red-hot in a clay tube, or in fine sand 

 in a small crucible, so as to keep them out of contact with the air. They may be 

 considered sufficiently free frouc polarity, if, when immersed in fine filings of soft 

 iron, there is no tendency to adhesion, or polar arrangement of the filings about 

 their extremities. 



N n 2 



