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Updated: May 7, 2025
A great variety of lamps was made of the platinum-iridium type, mostly with thermal devices to regulate the temperature of the burner and prevent its being melted by an excess of current. The study of apparatus for obtaining more perfect vacua was unceasingly carried on, for Edison realized that in this there lay a potent factor of ultimate success.
In his experiments with the platinum-iridium lamps, he had been working all the time toward the proposition of high resistance and small radiating surface, until he had made a lamp having thirty feet of fine platinum wire wound upon a small bobbin of infusible material; but the desired economy, simplicity, and durability were not obtained in this manner, although at all times the burner was maintained at a critically high temperature.
The hospital physician had bent over her at the words, and before Kennedy could go on interrupted: "This was not a snake bite; it was more likely from an all-glass hypodermic syringe with a platinum-iridium needle." Professor Rapport, priest of the Devil, advanced a step menacingly toward Kennedy.
Iridium is mixed with platinum in order to increase its strength and durability. The normal standards of the metrical system are made of platinum-iridium on account of its known immutabilty. In 1882, platinum stood 15 per cent. below its present value; but its increased employment for industrial purposes led to the subsequent improvement in price.
"There could be no snake in this case," I remarked. "The fangs of a serpent make two punctures, don't they; while here there was just the one scratch " "Of course there were no fangs when the deed was actually done," he rejoined, impatiently. "We've traced everything to the needle in the portieres and it is my belief that it was part of an all- glass hypodermic with a platinum-iridium point.
Another special instrument is a "dead beat" galvanometer which differs from the ordinary form of galvanometer in having no coils or magnetic needle. It depends for its action upon the heating effect of the current, which causes a fine platinum-iridium wire enclosed in a glass tube to expand; thus allowing a coiled spring to act on a pivoted shaft carrying a tiny mirror.
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