The microphone as a means of magnifying feeble sounds has been employed for localising the leaks in water pipes and in medical examinations.  Some years ago it saved a Russian lady from premature burial by rendering the faint beating of her heart audible.

Edison’s electric pen is useful in copying letters.  It works by puncturing a row of minute holes along the lines of the writing, and thus producing a stencil plate, which, when placed over a clean sheet of paper and brushed with ink, gives a duplicate of the writing by the ink penetrating the holes to the paper below.  It is illustrated in figure 93, where P is the pen, consisting of a hollow stem in which a fine needle actuated by the armature of a small electromagnet plies rapidly up and down and pierces the paper.  The current is derived from a small battery B, and an inking roller like that used in printing serves to apply the ink.

In 1878 Mr. Edison announced his invention of a machine for the storage and reproduction of speech, and the announcement was received with a good deal of incredulity, notwithstanding the partial success of Faber and others in devising mechanical articulators.  The simplicity of Edison’s invention when it was seen and heard elicited much admiration, and although his first instrument was obviously imperfect, it was nevertheless regarded as the germ of something better.  If the words spoken into the instrument were heard in the first place, the likeness of the reproduction was found to be unmistakable.  Indeed, so faithful was the replica, that a member of the Academy of Sciences, Paris, stoutly maintained that it was due to ventriloquism or some other trickery.  It was evident, however, that before the phonograph could become a practical instrument, further improvements in the nicety of its articulation were required.  The introduction of the electric light diverted Mr. Edison from the task of improving it, although he does not seem to have lost faith in his pet invention.  During the next ten years he accumulated a large fortune, and was the principal means of introducing both electric light and power to the world at large.  This done, however, he returned to his earlier love, and has at length succeeded in perfecting it so as to redeem his past promises and fulfill his hopes regarding it.

The old instrument consisted, as is well known, of a vibrating tympan or drum, from the centre of which projected a steel point or stylus, in such a manner that on speaking to the tympan its vibrations would urge the stylus to dig into a sheet of tinfoil moving past its point.  The foil was supported on a grooved barrel, so that the hollow of the groove behind it permitted the foil to give under the point of the stylus, and take a corrugated or wavy surface corresponding to the vibrations of the speech.  Thus recorded on a yielding but somewhat stiff material, these undulations could be preserved, and at a future time made to deflect the point of a similar stylus, and set a corresponding diaphragm or tympan into vibration, so as to give out the original sounds, or an imitation of them.