84 
verified the law of intensity at short distances and showed that the com- 
monly accepted law of inverse squares at short distances is quite inap- 
plicable. 
Having thus verified the theoretical law for short distances, the use of 
the telephone receiver as a standard for estimating the intensity at great 
distances from the source becomes possible. 
Tables of results summarizing the observations obtained at great dis- 
tances showed that the values of m required to reconcile the observed 
variations of intensity at each increase of distance with the theoretical 
law at great distances increased uniformly; and hence it is evident that 
the theoretical law can not be quite correct. In fact, there must be an- 
other cause of decay of intensity not taken account of in the theoretical 
discussion; and this other cause, whatever it is, produces results not in 
accord with an exponential law of variation. What this other cause is, 
the author does not undertake to say. 
THE INTENSITY OF TELEPHONIC SouNDsS. By A. WILMER DUFF. 
[Abstract.| 
If a sound of constant intensity act on a telephone transmitter, the 
intensity of the sound given off from the receiver will depend upon the 
total resistance, inductive and non-inductive, of the circuit. If the circuit 
include a resistance box and the total resistance be varied in a known 
way, the relative changes of current affecting the receiver can be calcu- 
lated. If, now, the receiver be held at varying distances from the ear, 
so that the sound emitted by it seems to the ear as loud as the sound 
heard directly from the distant source that acts upon the transmitter, then 
the variations in intensity of the sound given off by the receiver can also 
be estimated. (See preceding article.) 
By this method it was found that the intensity of sound emitted by 
the receiver varied roughly as the three-halves power of the current 
traversing the circuit. 
