CHAPTER IV. 



THE PIN-HOLE PROBE FOR SOUND PRESSURES. 



28. The pin-hole sonde, or probe. It was shown above that when the tele- 

 phone is detached but slightly at the connection-pipe t, figure 14, no pressure 

 effect is discernible in the U-tube. This, however, may be predicted if the 

 extremity of the pipe t is regarded as the end of an organ-pipe. Thus it seemed 

 worth while to determine the possibilities of sounding for pressure by aid of a 

 pin-hole valve at the end of a slender tube c (as in fig. 43), communicating 

 with the closed reservoir R' (IV) of the U-tube, the other, R, being open to 

 the atmosphere. The telephone T was therefore provided with a tubular 

 projection t, 12 cm. deep to the plate and about 0.75 cm. in the inner diameter. 

 This received the probe c (0 being at the end of an aluminum tube 2 mm. in 

 bore, 3 mm. in outer diameter, and about 20 cm. long) to different depths 

 with a view to exploring the pressures within. The side branch with cock C 

 and extra pin-hole 0' is useful below. Here C is to be kept closed. 



The probe used in the normal fashion (as in fig. 14) gave the results contained 

 in figure 44 when 500 and 1,000 ohms were successively put in the telephone 

 circuit. On comparing this with the usual pin-hole it was seen to be about 



Ocm, 4 



one-third as sensitive. Moreover, the details of the resonance have been oblit- 

 erated by the slender aluminum tube. A wider quill-tube will therefore be 

 preferable in general, though in the present case greater width would have 

 been permissible because of the small bore of the pipe t. Probably, however, 

 the decreased sensitiveness here observed is of no consequence, as there is 

 to be a minimum of vibration in the tube c when used for exploring or 

 sounding purposes. 



The results obtained on inserting the probe to different depths (marked on 

 the graphs) in the telephone pipe are given in figs. 45 and 46, compressions 

 being laid off positively downward in these figures. The curves are unexpect- 



