Dr. Tyndall on the Laws of Magnetisi'ri. S6^ 



effect this being taken as the equivalent of the attracting fdi'c6'.' 

 Although every pi'ecantion ^vas taken to secure accuracy, the 

 results thus obtained were not so satisfactory as might be desired ; 

 for even when the cuiTent remained constant, the weight uecessaiy 

 to separate the ball varied considerably in different experiments. 

 This was also the case when fine shot v.as poured upon the scale- 

 pan. In the laying on of weights or the pouring in of shot, it is 

 scarcely possible to avoid a vibration of the beam, which com- 

 municates itself to the ball in a series of slight jerks directed 

 upwai'ds. The weight upon the opposite scale-pan is ever ready 

 to take advantage of the slightest loosening of the ball occasioned 

 bj'^ these little jerks, and hence a separation may take place which 

 is not due to a fair overpowering of the magnet l^y gra^aty. 



6. A considerable number of experunents had been made in 

 this manner, when the thought occurred to me that the above 

 cause of disturbance might be removed by vising a variable mag- 

 net and fixed weight instead of a fixed magnet and a variable 

 weight. Hitherto I had excited the magnet by a certain current, 

 and added weights imtil the ball gave way. The method which 

 now suggested itself was to lay a certain weight upon the scale- 

 pan, and slowly to reduce the magnetic power until a separation 

 should take place. By means of the rheostat I had this com- 

 pletely in my power, and a few trials sufficed to demonstrate the 

 superioiity of this mode of experiment over the former. 



The method of proceeding being thus determined, a friend in 

 whose skill and fidelity I could rely* took charge of the reading 

 of the tangent galvanometer. I took up my place before the 

 magnet, Aveighted the balance, turned the rheostat, and observed 

 the moment of separation ; at the same moment the turning of 

 the rheostat ceased, and the angle of (Jeflection was called out. 



n\ l>-Mr;n', '•;■>' -4['2. •, >1lLi -mM ^, ,;-,,./■!,, ,: -„■: 



7. Proposition I. — To detei-mine the general relation between 

 the strength of an electro-magnet and the mutual attraction of 

 the magnet and a mass of soft iron, when both are in contact. 



It is necessary here to define what is meant by " the strength 

 of an electro-magnet." '-•m-m- m;. ,.11.^1,.! -^^W ^uuifus^ 



A magnetized needle set peqiendicular to the iliagi'ietic ineH- 

 dian is solicited towards that meridian by the earth's magnetism 

 with a certain force, H. 



A magnetized needle set parallel to the magnetic meridian, 

 operated on by a magnet set perjx-ndicular thereto, and so that 

 the axis of the magnet ]jroduced shall striki; the centre of the 

 needle, is HoYmtvAfrom tlic magnetic meridian by a certain force,//. 



When (he earth and the magnet act togethn- upon the needle, 

 * Mr. Tlionias Hirst. 



