28 A. G. Bell—An Induction Balance 
Prof. Hughes of London, England, Prof. Trowbridge of 
Harvard College, Prof. Rowland of Johns Hopkins University, 
and other authorities were consulted by telegraph as to the 
best theoretical form of induction balance for the purpose re- 
quired, while empirical experiments were being carried on 
under my direction in my laboratory at Washington by Mr. 
Sumner Tainter; in the electrical work-shop of Davis an 
Watts, in Baltimore, by Mr. J. H. C. Watts, and in the estab- 
lishment of Mr. Chas. Williams, Jr., in Boston, by Mr. Thomas 
A. Gleason. To test the influence of size of coil, an instru- 
ment was constructed in which the coils were no larger than 
the bullet for which we sought (as had been suggested by Prof. 
Newcomb), and experiments were also made with the enor-° 
? 
mous coils used by the late Prof. Henry in his researches upon 
induction, which were kindly lent to me for the purpose by the 
Smithsonian Institution, but neither the small nor the large 
coils produced more satisfactory results than those we had 
already obtained 3 
To test battery power, 20 enormous Bunsen elements, which 
had formerly been used to light the gas at the Capitol, were 
placed at my disposal by Mr. Rogers, electrician of the Capitol, 
but while great electro-motive force was evidently of use we 
derived no advantage from such a battery as this. 
ruptions of the primary circuit of all rates up to 600 interrup- 
tions per second,* and we found that the more rapid the rate 
of interruption the more distinct was the sound in the tele- 
phone. The hearing distance, however, was not proportionately 
increased. he automatic interrupter (shown in fig. 5), yield- 
ing about 100 interruptions per second, gave as good results as 
any, and was much more convenient. This interrupter was 
therefore afterwards used exclusively in our experiments. 
The theoretical form o coil suggested by Prof. John Trow- 
bridge was substantially the same as that proposed by Prof. 
Rowland, and is shown in fig. 6. 
The arrangement was quite sensitive to metal placed in the 
interior of the coil, but the hearing distance for a bullet exter- 
nal to the coils was no greater than before.+ 
Professor Hughes proposed to have two flat superposed 
coils wound on a single reel, so that the two coils should form 
* Mr. Sumner Tainter has since made an apparatus operating in a similar man- 
ner by means of which he has obtained as many as 4,000 interruptions of the 
-cireuit per second. 
e | lance obtained was not quite perfect, and we have since discovered 
that the insulation of the wires of one of the secondary coils was defective, 
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