566 TRANSACTIONS OF SECTION G. 
two distinctly different frequencies at which the free 1mpedance of a telephone 
is one and the same. 
If the ‘damped impedance’ of a receiver is subtracted vectorially from the 
‘free impedance,’ the difference may be called the ‘motional impedance’ of the 
receiver at the frequency considered; because it is the change in impedance 
produced by the motion of the diaphragm. It was discovered that the locus of 
the motional impedance of a receiver, as the frequency advances from 0 to 2,400 
cycles per second, is a circle passing through the origin of co-ordinates: In 
other words, if we plot the motional reactance as ordinates, to motional resistance 
as abscisse, step by step, as the frequency is increased, the points lie on a 
circle passing through the origin. The theory of the phenomenon has been 
worked out to a first approximation, and shows that a circular graph should be 
obtained, that the principal diameter lies at the resonant frequency of the dia- 
phragm, and that different receivers or types of receiver have different character- 
istic circular graphs.. From an examination of the circular graph of a receiver 
various mechanical and electrical constants of the instrument can be derived. 
8. The Electrical Measurement of Wind Velocities.* 
By Professor J. T. Morris. 
MONDAY, SEPTEMBER 9. 
Discussion on the Gas Turbine.” 
The following Papers were then read :— 
1. The Road Problem. By Sir Joun H. A. Macponaup, K.C.B., 
F.R.S.—See Reports, p. 373. 
2. The Acceleration of a Motor Car.2 By H. E. Wipers, M.A. 
In the first part of this paper was given a simple graphical method of pre- 
dicting from its design the acceleration and hill-climbing ability of a motor 
car. In the first place a torque-speed curve is constructed ; from this is deduced 
an acceleration-speed curve; and from this in turn is derived the acceleration- 
time graph. In working out these curves a 15-h:p. car is taken as typical. 
The next part of the paper contained an account of the methods in use for 
the experimental measurement of the acceleration of a motor car. The various 
advantages and disadvantages of these methods were discussed. The author 
then gave the actual acceleration curves for the above-mentioned car as obtained 
by the use of a recording accelerometer. Two cases were taken, (a) when 
starting as in a race, and (b) when starting as in ordinary running. The pre- 
dicted and actual curves were compared and the differences analysed. The 
author showed that the choking of the carburettor causes a material loss of 
acceleration on starting. 
The paper concluded with.a reference to the considerations which influence 
what may be termed ‘ideal accelération,’ and pointed out that this ideal is 
incapable of attainment with cars as at present constructed. 
1 See The Electrician, October 4, 1912. 2 Ibid. September 13, 1912: 
* See The Engineer, September 27, 1912. 
