Proceedings, &c.,for 1886. 269 



Mr. Campbell then read his paper, entitled '' On the Want of a 

 Uniform System of Experimenting Upon Timber," and the subject 

 was discussed by Mr. Skinner, Mr. Tisdall, the President, and Mr. 

 Campbell, the proposals in the paper meeting with general approval. 



Mr. Tisdall gave a description of the lake recently discovered at 

 Mount Wellington, in Gippsland, and exhibited some geological 

 specimens collected in its vicinity. 



The President exhibited some beautiful sectional models of steam 

 engines made by J. S. Schrceder, of Darmstadt, for the purpose of 

 illustrating lectures, &c. 



PKOCEEDINGS OF SECTION A, 1886. 



On the 31st March Professor Kernot read a paper on " Boiler 

 Explosions." 



On the 28th April Mr. John Booth, M.C.E., read the following 

 paper on "The Testing of Large Dynamos." 



On Some Expedients Employed in the Testing of Large 

 Dynamos, with a Note on the Calibration of Am-Meters 

 and Volt-Meters. 



By the term " large dynamos" I here mean to refer to those which 

 it would be inconvenient to test with their working load. No one, 

 for instance, would care to bring some hundreds of incandescent 

 lamps into the test-room every time a dynamo was to be tried. 



The work done in any circuit is ecj^ual to the product of the electro- 

 motive force (E.M.F.) by the resistance, from which, in conjunc- 

 tion with Ohm's law, it follows that if the machine is kept at such a 

 speed as to maintain the proper E.M.F. at its terminals, and the 

 current be passed through any resistance equal to that of the work- 

 ing load, the effect, in as far as regards the dynamo, is the same. 



The question that arises, then, is to find some suitable resistance, 

 in heating which the machine can expend its energy. A length of 

 wire is one of the first things that suggests itself, and, since resistance 

 and not conductivity is desired, and a considerable quantity has to 

 be employed, an inexpensive metal is naturally used — generally iron 

 wire, and m the form of thick spirals — to economise space. 



There are, however, serious drawbacks to this simple method. 

 Unless the wire is very heavy, the heating, due to the passage of a 

 large current, will be serious. On the other hand, if the wire is 

 thick, it has also to be long, and thus becomes both cumbersome 

 and expensive. Moreover, it has been found that the change of 

 temperature in the hot wire, due to passing draughts of air, affects 

 its resistance to an inconvenient degree, and entirely destroys any 

 hope of accurate work. 



