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ELECTRICAL STANDARDS. 159 
important in proportion as experiments become more exact, and it is best 
to have as little as possible to change in the end. 
12.—From Professor J. S. Ames, Johns Hopkins University, U.S.A., 
December 10, 1895. 
. . . I must say your proposal appeals to me in every way. The 
10° unit seems to me to be preferable to the 15° one. 
13.—From Professor H. L. Callendar, Professor of Physics, McGill 
University, Montreal, December 5, 1895. 
I entirely agree that it would be a very great improvement to adopt 
an absolute unit in place of the present various and uncertain units based 
upon the peculiar properties of water. I think, however, that it would be 
better to connect it more simply and directly with the system of electrical 
units, and to use only names which are already familiar to all engineers, 
than to attempt to retain a close approximation to the value of any of the 
old specific heat units, which are essentially arbitrary. 
The following are the names of the series of thermal units which I 
should be inclined to suggest as being already familiar in practice :— 
1. The thermal watt-second, or ‘Joule,’ defined as being equivalent 
to 10’ c.g.s. units of work. <A rider might be added to the effect that, 
according to the best determinations, this unit is approximately equal to 
1 of the gramme degree centigrade at 10° C. 
2. The thermal watt-hour, which would be equivalent to 3,600 
Joules, and would therefore be of a similar magnitude to the kilogramme 
degree centigrade, which is so largely used in the thermo-dynamics of the 
steam-engine. The watt-hour, in fact, would be exactly ths of the kilo- 
gramme degree centigrade at some temperature in the neighbourhood of 
10° C. 
3. The thermal kilowatt-hour, or simply kilowatt-hour, which, as 
the Board of Trade unit of electrical energy, is already so familiar and 
useful for the commercial measurement of large quantities of energy. 
In connection with the latter unit it may be remarked that it would 
be a great advantage if engineers could be induced to adopt the kilowatt 
as their unit of mechanical power in place of the horse power. The latter 
unit differs from the ‘cheval-vapeur,’ and being based upon the foot-pound 
has different values in different latitudes. For the order of accuracy 
generally attainable in steam-engine work, it would, as a rule, be sufficient 
to take the horse power as being #ths of the kilowatt power. 
For steam-engine work undoubtedly one of the most important units 
at present in use is the British thermal unit, or pound degree Fahrenheit. 
It happens that the watt-hour is very nearly equal to 3-400 B.T.U. 
The reduction of the latter to watt-hours may be very readily effected by 
multiplying by 0-3 and then reducing the result by 2 per cent. 
It would seem, on the whole, not improbable that the simple adoption 
of al) the familiar units of electrical energy, with the prefix ‘thermal,’ if 
necessary, as our absolute units of heat, would result in a more general 
agreement and a greater simplification of expression than any attempt to 
re-define one of the older units in terms of the absolute system. The 
