34 A. G. Bell— An Induction Balance 
This led me to try the effect of a conical primary coil A with 
the secondary B at its apex, as shown in fig. 9, but the hearing 
distance for a bullet was only 3:5, 
Fig. 8. 
B 
Hess 
eo: BK 
.B 
Singularly enough Mr. J. H. ©. Watts, in Baltimore, had 
independently arrived at a very similar form of coil, and wit 
the instrament shown in fig. 10 he had obtained at one time a 
hearing distance of 75™ (or 3 inches), but from some cause 
unable subsequently to reproduce the 
not ascertained he was 
effect. 
