BURGLAR ALARM

New Zealand Herald, Volume LXIX, Issue 21274, 30 August 1932, Page 11

BURGLAR ALARM

TELL-TALE LIGHT BEAM

USE OF INVISIBLE RAY Faced with the possibility of springing a hidden alarm so sensitive that a puff of tobacco smoke will set it off, tho boldest bank robber might think twice before setting out to -practise his craft. Two Aucklanders, who had previously obtained a photo-electric cell and constructed an alarm with it, recently imported a complete apparatus as manufactured commercially in America. This they find to bo considerably better than their own amateur effort, and it has proved most interesting to their friends. The component parts arc a small projector containing ar\ electric light bulb, a photo-electric cell in a metal case and a transformer. The projector is trained so as to cast a narrow beam of light down a tube of rectangular section and about six inches long, projecting from the case which houses the cell. If by any means the light is obscured, the cell-instantly breaks an electric circuit. '1 he purpose of the transformer is to provide a circuit of higher voltage which can bo used to operate an alarm or other mechanism through a relay. Trials show that Hie alarm wjll work across a space of fully 100 ft., using the ordinary white light, of the electric bulb. It works quite satisfactorily at shorter distances when a piece of red glass, like that of a dark-room lamp, is inserted into tho projector. As a matter of a filter passing only infra-red rays, or "invisible light," could be used with tho same result. As a burglar-detector, the apparatus can lie made as nearly infallible as any mechanism can be. The slightest interruption of the beam of light sets it So does interference with the mechanism. The latter, however, can bp placed in such positions and so masked that tho astutest burglar could not find it, and certainly not without betraying his presence through the alarm. Both projector and cell could be hidden in walls or furniture and their small apertures masked. A narrow beam of red light is invisible in tho darkest room except where it impinges upon a reflecting surface, and if an infra-red filter is used it becomes wholly invisible. Willi battery current the apparatus would still work after the burglars had cut off tho electricity at the main.