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These experiments mark the discovery of electro-magnetism and begin 
one of the most important eras in electrical discovery. and one in which 
many eminent authorities participated. Among the many advances 
mnmay be mentioned the experiments by Henry on the relative effects of dif- 
ferent windings on the strength of an electro-magnet. He deduced the 
fact that the magnetizing action might be increased either by increasing 
the number of windings, the current remaining the same; or by increasing 
the current, the windings remaining the same. He pointed out the applica- 
tion of this to intensity and quantity arrangement of the battery, and also 
the importance of the intensity winding for the transmission of magnetiz- 
ing power to a distance, as in telegraphy. The increased effect due to in- 
creasing the number of windings on the coil of a galvanoscope had been 
previously pointed out by Schweigger, and the discovery is embodied in 
Schweigger’s galvanoscope. 
In 1821, Faraday began his researches and many important discoveries 
were made by him. The main guiding idea in Faraday’s work was the pos- 
sibility of obtaining electricity from magnetism, and in general the dis- 
covery of the inter-relation between the two. In this connection, Aragos 
discovery of the rotation of a copper disc by the rotation of a magnet 
above it is of great importance, because among other things Faraday set 
himself to explain this. The result was the discovery of the commuta- 
torless dynamo or Faraday disc. In view of modern developments, prob- 
ably the most important of Faraday’s discoveries was that of the pro- 
duction of a current in a circuit when a current is either established or 
varied in strength in an adjacent circuit. This was followed by the dis- 
covery that relative motion of two circuits, one of which carried a current, 
poduced a current in the other, and that the motion of a magnet in the 
neighborhood of a circuit produced a current in the circuit. Another im- 
portant discovery by Faraday was that of the quantitative laws which 
govern electrolytic decomposition, thus giving us our electro-chemical 
equivalents. 
At this time Lenz was led by experiment to the discovery of his cele- 
brated law of inductiou, namely, that the current produced always in turn 
produces forces tending to oppose the change. For example, if a current 
be induced in a coil by bringing a magnet toward it, the mutual action be- 
tween the magnet and the current is to oppose the magnet’s approach. 
This is important when looked at from the point of view of the conservation 
