ADDRESS OF PROF. A. M. MAYER. 485 



weight of severa] hundred pounds. When the lifter however is 

 detached, nearly all the magnetism disappears." 



( )n ;i recent visit to the College of New Jersey by the electrician 

 Mr. Frank L. Pope, he examined this magnet. "There," he says 

 in his admirable and justly appreciative eulogy on Henry, "there, 

 ton, was the reversing commutator or pole-changer, a device first 

 invented by Professor Henry, with which he was accustomed to 

 delight and astonish his pupils, by suddenly reversing the polarity 

 of his large magnet, causing it to drop its armature and seize it 

 again before it had passed beyond the sphere of attraction, a prin- 

 ciple which we see exemplified in every stroke of the neutral relay 

 of the quadruplex telegraph of to-day." 



We will new return to Henry's study of the properties of his 

 intensity magnet. This magnet was formed of a piece (if iron one- 

 fourth (if an inch in diameter, bent into the U form and wound with 

 s feet of insulated wire. His batteries Avere two, — one formed of a 

 single element with a zinc plate 4 inches by 7, surrounded by copper 

 and immersed in dilute acid; the other, a Cruikshank's battery, or 

 trough, with '_'•"> double plates. The plates of this battery were 

 joined in series and altogether had exactly the same surface of zinc 

 as that in the single-cell battery. 



The magnet was now connected directly to the single cell. The 

 magnet held up 72 ounce's. Then 530 feet of number 18 copper 

 wire led the current from the cell to the magnet; it now sup- 

 ported only two ounces. Five hundred and thirty feet more of the 

 wire were introduced into the circuit and then the magnet held but 

 one an nee. In these facts Henry faced the same results as con- 

 fronted Barlow five years before, and caused Barlow then to say: 

 "In a very early stage of electro-magnetic experiment-, it had been 

 suggested [bv Laplace, Ampere, and others] that an instantaneous 

 telegraph might be established by means of conduct ing-wires and 

 compasses; - - - but I found such a sensible diminution with 

 only 200 feet of wire, as at once to convince me of the impractica- 

 bility of the scheme:" and such, at that day, seemed to be the 

 common opinion of men of science. But this opinion is presently 

 to be shown by Henry to be ill founded, by reason of the ignorance 

 of the relations which have of necessity to exist between the kind of 



