SECTIONAL TRANSACTIONS.—G. 49 , 
of size and weights of electric motors and gear wheels. Description of opera- 
tion of electric generators and motcrs—magnetic forces; revolving fields. Model 
showing action of forces and speed-reduction features. Control. Operation of 
_ excitation and reversing switches. Analogy with motor-car control. Model to 
_ show operation of control. 
; Further considerations of electric transmission—efficiency, reliability, repairs, 
ventilation. 
Effect of electric transmission on turbines and propellers, superheated steam, 
reversing blades. Use of double-ratio transmission for naval vessels. 
Electric transmission for Diesel-engine ships. Simplicity of starting, sub- 
division of engines; use of most suitable speeds. 
Examples of electric ships and experience obtained with them. 
21. Mr. R. L. Morrison.—Conversion from Alternating Current to 
Direct Current by means of Mercury Are Rectifiers. 
The need for a simple stationary converter that can be compared with the 
static transformer. Valve action of the mercury arc, with fundamental con- 
siderations underlying it. The efficiency of this form of converter, high for 
high-pressure D.C. conditions, thus making it eminently suited to main-line 
electrification. Single and polyphase rectifiers with wave forms. Special trans- 
ormer enabling both halves of alternating wave to be utilised. Construction 
of large rectifiers. Sealing against atmosphere. Anode material and its effect 
on the continuous operation of the plant. Main auxiliaries—i.e. vacuum pump, 
ignition converter, &c. Sizes at present manufactured. Operation in conjunc- 
tion with other types of converter. Overloading. Rectifiers specially for high- 
tension D.C. work, with details of those at work on the Midi line of France. 
Upkeep. Automatic control. Advantages and general information. 
22, Dr. T. F. Watu.—Squirrel-cage Induction Motor with High 
Starting Torque and Low Starting Current in the Line. 
The rotor conductors are built up as composite bars as follows: A central 
copper rod is surrounded by a steel sheath, and the outside of the steel sheath 
is copper-plated to a suitable thickness. 
Each rotor conductor is thus in effect a simple form of transformer, the ° 
central copper rod being the primary winding, the steel sheath being the 
magnetic core, and the outer copper-plating the secondary winding. 
In accordance with a well-known result in the theory of transformers, the 
desired result is obtained that the resistance of the rotor winding is automatically 
increased at starting, whilst when normally running it has the same value as 
for a standard type of squirrel-cage motor. 
23. Prof. W. M. Tuorntcn.—The Mechanism of Gas Ignition. 
The two modes of spark ignition used in engineering are by disruptive 
discharge at high voltages and by the arc formed at the point where a circuit 
is broken. The former is a well-known case of ionisation by collision, but 
ignition does not depend solely upon the sparking voltage. There is for each 
inflammable mixture a certain intensity of spark necessary for ignition. Below 
this it is possible to pass sparks indefinitely without explosion resulting. A 
certain rate of production of ions is necessary, which bears little or no relation 
to the total energy dissipated in the spark. Ignition occurs when a critical 
rate of electrical activation of the combining gases is reached. 
The case of the momentary arc at break is less simple. That it is mostly 
ionic and not thermal has been shown by a long series of observations, in which 
wide but regular differences between the ignitions of various gases under varied 
electrical conditions have been found. Kecent observations on the ignition of 
hydrogen-air mixtures give more direct evidence of its nature. Working with 
alternating currents at a frequency of 250 a second and a circuit voltage of 85, 
the following results were regularly obtained: After breaking a 10-ampure 
circuit fifty times in air alone, iron poles being used, ignition of a 35 per cent. 
hydrogen-in-aee mixture occurred with a circuit current of 0.4 ampere. After 
