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XL Experimental Researches in Electricity. — Twenty-ninth Series. By Michael 
Faraday, Esq., D.C.L., F.R.S., Fullerian Prof, Cheni. Royal Institution, Foreign 
Associate of the Acad. Sciences, Paris, Ord. Boruss. Pour le Mdrite, Eq., Memh. 
Royal and Imp. Acadd. of Sciences, Petershurgh, Florence, Copenhagen, Berlin, 
Gottingen, Modena, Stockholm, Munich, Bruxelles, Vienna, Bologna, 8^c. ^c. 
Received December 31, 1851, — Read March 25 and April 1, 1852. 
§ 35. On the employment of the Induced Magneto-electric Current as a test and 
measure of Magnetic Forces. 
3177. The proposition which I have made to use the induced magneto-electric 
current as an experimental indication of the presence, direction and amount of mag- 
netic forces (3074.), makes it requisite that I should also clearly demonstrate the 
principles and develope the practice necessary for such a purpose ; and especially 
that I should prove that the amount of current induced is precisely proportionate to 
the amount of lines of magnetic force intersected by the moving wire, in which the 
electric current is generated and appears (3082, 3109.). The proof already given is, 
I think, sufficient for those wffio may repeat the experiments; but in order to accu- 
mulate evidence, as is indeed but proper in the first announcement of such a propo- 
sition, I proceeded to experiment with the magnetic power of the earth, which pre- 
sents us with a field of action, not rapidly varying in force with the distance, as in 
the case of small magnets, but one which for a given place may be considered as 
uniform in power and direction ; for if a room be cleared of all common magnets, 
then the terrestrial lines of magnetic force which pass through it, have one common 
direction, being that of the dip, as indicated by a free needle or other means, and are 
in every part in equal proportion or quantity, i. e. have equal power. Now the force 
being the same everywhere, the proportion of it to the current evolved in the moving 
wire is then perhaps more simply and directly determined, than in the case where, a 
small magnet being employed, the force rapidly changes in amount with the distance. 
^ i. Galvanometer. 
3178. For such experimental results as I now propose to give, I must refer to the 
galvanometer employed and the precautions requisite for its proper use. The in- 
strument has been already described in principle (3123.), and a figure of the conductor 
which surrounds the needles, given. This conductor may be considered as a square 
copper bar, 0'2 of an inch in thickness, which passes twice round the plane of vibra- 
tion of each of the needles forming the astatic combination, and then is continued 
outwards and terminates in two descending portions, which are intended to dip into 
MDCCCLII. T 
