ADDRESS OF PROF. W. B. ROGERS. 85 



face with but few elements, that is, a quantity-battery, should be 

 used ; and that in the latter case the effect was greatly increased by 

 using many separate short coils to inclose the magnet, each connected 

 with the galvanic source, or in place of these a single thicker wire, 

 forming thus what he termed a " quantity-magnet." 



It was in this stage of his researches that, in 1831-'32, Henry 

 produced a machine moved by electro-magnetism, and exhibited in 

 the Albany Academy the memorable experiment of transmitting 

 signals by means of his electro-magnet through more than a mile 

 of wire, and soon after pointed out the application of the principles 

 shown to the transmission of intelligence to a distance. This was 

 undeniably the first example of what was virtually an electro- 

 magnetic telegraph, and furnished a scientific foundation for those 

 multiplied inventions which in later years have made the electro- 

 magnetic telegraph co-extensive with the civilized world. 



We may not here consider the various claims of the ingenious 

 inventors who in later years originated the numerous details of 

 practical telegraphy. It was a period in which discovery and 

 invention were, as it has been said, "in the air;" and it would be 

 impossible to assign to any, even the most illustrious contributor 

 to the result, his own precise share in the general progress. 



Not pausing to make further applications of the discoveries re- 

 ferred to, so suggestive of great practical use, and not for a moment 

 considering the profitable return which might be secured from them, 

 Henry, in the spirit of a true lover of science, continued his 

 investigations in the same general field, and after his removal to 

 Princeton made other and larger additions to the store of electrical 

 knowledge. Here, repeating an earlier experiment, he made the 

 important discovery of the reaction of the current upon itself, caus- 

 ing what is called the extra-current, and carried on the very original 

 investigations which revealed the existence and the laws of induced 

 currents of successive orders, which, for their novelty, ingenuity, 



