476 RECORDS 



Summary of Papers. 



Professor Hallock stated that the first regular overtone of a 

 tuning fork is about two and a half octaves above the funda- 

 mental, but Lord Rayleigh pointed out that when the amplitude 

 of the vibration became so great that the restoring force was no 

 longer proportional to the displacement, the octave appeared, as 

 indicated by theory. Lord Rayleigh recognized the presence 

 of the octave with his ear and by the use of a resonator. Pro- 

 fessor Hallock obtained direct evidence of this effect by means 

 of a photograph of a manometric flame, the capsule of which 

 was resting against the prong of the fork. 



In a paper on "Specific Gravity Weighings," Professor Hal- 

 lock spoke of a number of points in which the ordinary opera- 

 tions could be improved. It is very convenient to use the 

 principle of the Jolly balance in which there is a pan always 

 immersed to hold the body when weighing it in water. The 

 effect of capillarity on the supporting wire which at best lessens 

 the sensibility of the balance, can be avoided by sending a series 

 of little waves across the surface of the water while weighing. 

 To get rid of bubbles in little corners of irregular bodies, these 

 bodies may be held under the tap at the sink and moistened 

 with water before immersion or, still better, they may be moist- 

 ened with alcohol and then with water before immersion. 



Professor M. I. Pupin described a new faradmeter, which he 

 had devised, an instrument for measuring the capacity of a con- 

 denser. This instrument is essentially a VVheatstone's bridge, 

 using alternating currents, in which one leg of the bridge con- 

 sists of two resistances in series and the other leg consists of 

 two capacities in series, one of the two being that of a standard 

 condenser, the other being the unknown capacity to be meas- 

 ured. In the bridge connecting the two points, one between 

 the two resistances, the other between the two capacities, is a 

 telephone. If the two separate circuits, each containing one of 

 the two capacities, are arranged so that the capacity reactance 

 is by far the greatest part of the impedance in that circuit, then 

 silence in the telephone will be obtained when the two resist- 



