By continuing to enlarge the diameter, a coarse, hollow, drum-like effect was produced, until when the diameter became very large, the sound resembled that one hears when the head is inside a barrel, and was accompanied with a reverberating sound. By reversing the above conditions—that is, by keeping the diameter constant, and varying the thickness—it was found that with a very thin plate the drum-like sound was produced; by gradually increasing the thickness this effect passed off; then followed distinct articulation, until at a certain increase of thickness the peculiar nasal quality again developed itself.
In practice it has been found desirable, in establishing speaking communication between two distant places, to employ two telephones instead of a single one; one being applied to the mouth and the other to the ear during a conversation.
With one telephone it was no unusual occurrence for confusion to arise in consequence of the two speakers talking or listening at the same time.
So faithful is the transmission by the telephone of every variety of sound, that Mr Preece states, when in telephonic communication with Prof. Bell, through a quarter of a mile he has heard him “laugh, sneeze, cough, and, in fact, make any sound the human voice can produce.” It must be borne in mind, however, that the transmitted speech can only be distinctly heard in the immediate vicinity of the receiving apparatus; the keenest hearing fails to detect it at the distance of little more than a foot away. Hence, when a message is expected, the recipient has to place his ear to the mouthpiece of the instrument, and use it as an ear-trumpet.
A circumstance tending to impair the satisfactory working of Bell’s telephone is, that the line wire to which the ends of the coil are attached becomes inductively affected by the currents of electricity passing through the parallel and contiguous telegraph wires, the effect, on a line where there is an active transmission of telegraphic messages, being that the telephone “emits sounds that are very like the pattering of hail against a window, and which are so loud as to overpower the effects of the human voice.”[228]
[228] Preece.
This inconvenience can, however, it is stated, be remedied.
If all the arrangements of the instrument were perfect, there should be no limits to the distance through which speech could be conveyed by the telephone. Professor Bell says that in laboratory experiments “no difficulty has been found in using an apparatus of this construction through a circuit of 6000 miles;” and that he had found it act efficiently between New York and Boston, a distance of 258 miles, subject to the condition that the neighbouring telegraph wires were not in action.
Mr Preece has carried on conversations between Dublin and Holyhead, a distance of 100 miles.
Two useful applications of the telephone are recorded by Professor Bell, the one its employment in connection with the diving bell; the other as a means of communication between those above and below ground in mines. It has been largely adopted in extensive factories and in commercial houses both in America and in this country, supplementing, because of its much greater simplicity and easy application, the electric telegraphs previously in use in such establishments.