1883.] 



On a Magnetic Balance. 



169 



to the right or left according to the polarity of its magnetism, and 

 with a force proportional to its power. If we place on the opposite 

 side of the needle at the same distance a wire possessing similar 

 polarity and force, the two are equal, and the needle returns to zero, 

 and if we know the magnetic value required to produce a balance we 

 know the value of both. In order to balance any wire or piece of 

 iron placed in a position east and west, a magnetic compensator is 

 used, consisting of a powerful bar magnet free to revolve upon a 

 central pivot, placed at a distance of 30 or more centims., so as to be 

 able to obtain delicate observations. This turns upon an index, the 

 degrees of which are marked for equal degrees of magnetic action 

 upon the needle. 



A coil of insulated wire, through which a feeble electric current is 

 passing, magnetises the piece of iron under observation, but as the coil 

 itself would act upon the needle, this is balanced by an equal and 

 opposing coil on the opposite side, and we are thus enabled to observe 

 the magnetism due to the iron alone. A reversing key, resistance coils, 

 and a Daniell cell are required. Great care must be taken so that the 

 electromotive force remains a constant, as a small variation in the 

 electromotive force gives large variations in the readings, and many 

 important details of construction are required, in order that it shall 

 give perfect readings for extremely small magnetic foi*ce. Still 

 greater care is required that each specimen of iron or steel shall be 

 annealed to its maximum. 



Several methods of observation have been employed with the 

 magnetic balance, the usual one being the one described, but inte- 

 resting results are attained by observing the influence of earth's 

 magnetism alone on the iron or steel, or we may magnetise all speci- 

 mens to the same value, and note the amount of current required. 

 We may observe the remaining magnetism after the cessation of the 

 current or the influence of a weak current after the passage of a 

 strong magnetising force. These are more applicable to researches 

 upon the cause of magnetism. 



By means of this instrument I have tested sixty varieties of iron 

 and steel, mostly in the form of wires, a wire 1 millim. diameter, 

 10 centims. long, being the standard used. In all comparative expe- 

 riments we require one standard form, to which all the rest must be 

 similar in form and size ; at present, we cannot readily compare a 

 square or flat bar with a piece of wire, but if all pieces have the same 

 form, and all are annealed to the same standard, then any difference 

 observed between them must be due to their comparative softness, 

 from which we can deduce its quality and place on the line from soft 

 iron to cast steel. 



Annealing not only produces softness and consequent molecular 

 freedom, but it entirely frees it from all strains previously introduced 



n 2 



