436 SECTIONAL TRANSACTIONS.—G. 
made to include as far as possible the most interesting points in recent design— 
the first half of the lecture dealt with the production of a.c. energy, the second 
part with c.c. energy. 
14, Mr. Joun Scorr-Taacart.—Iwo New Negative Resistance 
Devices for use in Wireless Telegraply. 
The paper opened with a short account of the uses of negative resistance, 
the chief being the production of continuous oscillations for use in wireless 
telegraphy. The author then described two new thermionic valve devices by 
means of which negative resistance characteristics may be obtained. The first 
device, the ‘ Negatron,’ has attained commercial importance as a generator of 
continuous oscillations for use in wireless telegraphy. The principle involved 
is the redistribution of electrons passing between a cathode and two anodes. 
By increasing the potential of one anode the electron current is diverted by 
means of an electrostatic field towards the other anode. The first anode current, 
therefore, decreases. In the second device two three-electrode valves are used 
and the circuital arrangments are such that when a positive potential is applied 
to the anode of one of the valves a negative potential is given to the grid, 
thus giving negative resistance characteristics. 
15, Dr. T. F. Watu.—The Long-distance Transmission of Electrical 
Energy Generated by Means of Tidal Power. 
The problem of the generation and transmission of electrical energy obtained 
from tidal power involves serious difficulties on account of the widely varying 
speeds of the turbines working under a variable head of water. For example, 
the proposals of the Ministry of Transport for the Severn scheme comprise the 
use of special direct-current generators driven by the turbines, the direct current 
being then converted into alternating current by means of rotary converters and 
then transformed into high-tension current for transmission. Such a scheme 
involves a very large number of machines and tends to make the proposal pro- 
hibitive from the point of view of capital outlay. The purpose of this paper was 
to describe a scheme in which the difficulty due to the variable speed of the 
turbines is obviated, the scheme being based on the characteristic phenomena 
of quarter-wave transmission. 
Wednesday, September 14. 
16, Dr. S. F. Barcuay.—Modern High-speed Centrifugal Pumps. 
The fourteenth-century records of the French Academy contain a reference 
to a primitive form of centrifugal pump, and it is probable that the principle 
involved is quite an ancient discovery. The foundation of the modern form 
of turbine pump was laid in 1875, when Osborne Reynolds invented the 
stationary guide vanes. The present-day development is due mainly to the 
industrial evolution of the electric motor with its demand for high angular 
speeds. The efficiency of the centrifugal pump attains a high value only when 
worked at the full-rated output. The advantages, however, of the compactness, 
simplicity, and low first cost of pumps of this kind as compared with displace- 
ment pumps are so marked that they are often employed for intermittent work 
with which high efficiency is not possible. Centrifugal pumps are suitable for 
use under a very wide range of conditions; they are used for lifting water 
against a head of only a few feet or against a head of several thousand feet, 
and for small or large volumes. Centrifugal pumps are constructed that require 
only a fraction of a horse-power to drive them, and at the other extreme there 
are pumps that require several thousand horse-power. 
17. Dr. J. S. Ownns.—Haperiments on Air-Lift Pumping. 
This paper described experiments on some of the factors which govern the 
efficiency of air-lift pumps; it dealt especially with so-called slippage losses 
arising from relative motion of air-bubbles through the water; with the relation 
of size of bubbles to velocity of rise; the conditions governing size of bubbles; 
the effect of diameter of orifice delivering air upon the size of bubbles produced ; 
——— ee lee 
