Fig. 67.
Fig. 68.
§ 64. We now have to consider those contrivances by means of which it is possible for an attendant to
know when a single bell is actuated by a number of pushes in different rooms, etc., from whence the signal emanates. These contrivances are known as indicators. Indicators may be conveniently divided into 3 classes, viz.:—1st, indicators with mechanical replacements; 2nd, those with electrical replacements; and 3rdly, those which are self replacing. Of the former class we may mention two typical forms, namely, the ordinary "fall back" indicator, and the drop indicator. All indicators depend in their action on the sudden magnetisation of an electro-magnet by the same current that works the electric bell at the time the call is sent. To understand the way in which this may be effected, let the reader turn to the illustration of the Relay ([Fig. 66]), and let him suppose that the pillar D, with its accompanying rectangle B C, were removed, leaving only the electro-magnet G, with its frame and armature A. If this armature holds up a light tablet or card, on which is marked the number of the room, it is evident that any downward motion of the armature, such as would occur if the electro-magnet were energised by a current passing around it, would let the tablet fall, so as to become visible through a hole cut in the frame containing this contrivance. It is also equally evident that the card or tablet would require replacing by hand, after having once fallen, to render it capable of again notifying a call. [Fig. 67] shows the working parts of one of these "drop" indicators, as sent out by Messrs. Binswanger. In another modification, known as Thorpe's "Semaphore Indicator," we have a most
ingenious application of the same principle in a very compact form. In this ([Fig. 68]), the electro-magnet is placed directly behind a disc-shaped iron armature, on which is painted or marked the number of the room etc. (in this case 4); this armature is attached by a springy shank to the drop bar, shown to the left of the electro-magnet. In front of the armature is a light metal disc, also pivoted on the drop bar. This engages in a catch above, when pushed up so as to cover the number. When pushed up, the spring of the armature retains it in its place so that the number is hidden. When the current passes around the electro-magnet, the armature is pulled toward it, and thus frees the covering disc, which therefore falls, and displays the number. The ordinary form of "fall back" indicator (a misnomer, by the way, since the indicator falls forwards) is well illustrated at [Fig 69]. Here we have an ordinary electro-magnet A, with its wires w w' standing over an armature B attached to a spring C, which bears on its lower extremity, a toothed projection which serves to hold up the short arm of the bent lever D, which supports the number plate E. When the electro-magnet A is energised by the current, it pulls up the armature B, which releases the detent D from the tooth C; the number plate therefore falls forwards, as
shown by the dotted lines, and shows itself at the aperture E´, which is in front of the indicator frame. To replace the number out of sight, the attendant pushes back the plate E, till it again engages the bent lever D in the tooth C. This replacement of the number plate, which the attendant in charge is obliged to perform, gives rise to confusion, if through carelessness it is not effected at once, as two or more numbers may be left showing at one time. For this reason, indicators which require no extraneous assistance to replace them, are preferred by many. Indicators with electrical replacements meet in part the necessities of the case. This form of indicator consists usually of a permanent bar magnet pivoted near its centre, so that it can hang vertically between the two poles of an electro-magnet placed at its lower extremity. The upper extremity carries the number plate, which shows through the aperture in the frame. This bar magnet is made a trifle heavier at the upper end, so