Vol.. 8, 1922 PHYSICS: C. BARUS 163 
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having a resistance of 400 ohms and a sensitivity of 6.02 X 10~^^ amps./ mm. 
as now used. In this work it has been critically damped. The deflec- 
tions for illuminations such as used ran as high as 100 cm. The contacts 
tised were very heavy strips of copper, about 2'' by 10" and Vs'' thick; 
they were blackened by smoke from burning naphthalene except at the 
point of contact, and were pressed firmly against the crystal by weights 
placed on them. The crystals were natural and several specimens were 
used, obtained from the following localities: Frieberg, Saxony, Germany; 
Schneeberg, Saxony, Germany; Batopilas, Mexico; and Arispe Sonora, 
Mexico. 
POSITIVE AND NEGATIVE PINHOLE RESONATORS'' 
By Cari^ Barus 
Department of Physics, Brown University 
Communicated, May 11, 1922 
Improvement of the Pinhole Resonator. — The plan immediately suggesting 
itself was the trial of an adjustable needle valve pinhole. The apparatus 
was made of V4 i^^^h brass tube, with a conical point carrying the pinhole 
at its end. The tube was closed by a long nut in which a waxed screw 
terminated by the needle was movable. The pinhole could thus be completely 
closed, or opened in any degree by approaching or withdrawing the screw. 
The new pinhole was inserted into the tuned resonator, as usual. After 
many trials, nothing was obtained with this promising apparatus, until 
the needle was completely withdrawn. In fact the best pin holes obtained 
for the resonator, were made from glass quill tubes drawn to a blunt closed 
cone. It is necessary to open and enlarge the hole by grinding; or to 
close the conical end by gentle fusion and regrinding when cold. 
To throw further light on this intricate subject, it seemed advisable to 
operate with pinholes in soft sealing wax, as these could be more easily 
shaped. In this way much information was obtained. A thin sheet of 
wax was spread on thin-paper closing the end of the quill tube. When the 
wax was punctured by a needle from the inside of the tube, producing a 
conical hole as exaggerated at 5 in figure 1, the fringe deflections were 
positive and reached a maximum with the right size of hole. But when 
the thin wax sheet was punctured from the outside of the quill tube (sug- 
gested at r fig. 1.), the deflection obtained was to the same degree negative. 
In other words, in passing from the salient pinhole to the reentrant pin- 
hole, acoustic pressures pass to acoustic dilatations; or the pressure excess 
is on the reentrant or concave side of the conical pinhole. It follows there- 
fore that a cylindrical pinhole must be inactive, if used with a resonator. 
