Page:Popular Science Monthly Volume 89.djvu/772

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��Popiihir Science Mnnilihj

��iinisiblo, l)iit by no means silent drama which mis;ht follow such a catch in a home. 'i"he cursing of the burglar, the screams of h\sterical women and cr\'- ing children, the excited father, and a "drag-]iini-onl-li\-lhe-police-rinale"were
 * )Iaiiil\' heard.

Tlu- |)rin(i|)al parts of the apparatus, which we lia\e callcil the eye and the brain, are a selenium cell and a sensitive rela\-; the n e r v o u s energ\' is suj)plied by a battery of (ells, which an- connected in series with them as shown in the ii|)perdiagram. If onl>' an alarm is desired, an electric bell and battery may be connected to the loial cir- cuit terminals of the sensitive relay. This bell will ring instantly w h en a light strikes the selenium cell, and will continue ringing as long as the cell is ilhimiiialfd. li may l)e placed in a sleeping apartment at a distance from tile room to be protected, so that the burglar will be unaware of the fact lh.it his light has sent in .in .il.irin. .\ device of this ii.ilini- \\(aild be valu- able fur the |)rolection of vaults.

The seliiiiiiMi (ills may be purchased Irom scientific su[)- ply houses at a cost of about ii\'e dollars each. The relay should be as sensitive as possible; a good polarized relay may suftice but a galvanometer relay is preferable.

The battery should consist of a suliicieiit number of dry cells (these m.i\- be (jf the small llashlight t\pe) to iiairly cause the closing of the relay contacts when the selenium cell is in the dark, and when the b.u k spring of the relay is iu sulhcient tension to prevent slicking of ihe contacts after the light r.ivs ;uc obstru( led. When the ( ell i> illuniiii.i-

���Above: The dry cells connected in series. Below: The plan of the connections when several pieces are to be set off simultaneously

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��The selenium cell is the eye of the crea- ture which when illuminated con- tacts the relay

��ted the relay should close jiromptly, and when the light is removed the back spring should pull the contacts apart without hesitation. In general the larger the number of batteries the more sensitive v\-ill the apparatus be, but with ordinarv' selenium cells the normal current should not exceed a few thou- sandths of an ampere. If the current is too large the temperature of the cell may rise to the point at which selenium begins to melt ; this will destroy the use- fulness of the cell. From the relay, acting as the brain, we may- lead connections to whatever ap- paratus we desire to be actuated when the seleni- um cell is stimula- ted by light.

When several pieces of appa- ratus are to be set off simultaneously, for example the apparatus before described, a connec- tion arrangement, such as that shown in the diagram, should be used. One huiulreil and ten volts are suggest(?d since this is the voltage of mo.-^t lighting sv'stems, and because a solenoid sutiicienfly l.u'ge to pull the trigger of a revolver will operate best on that voltage without addition- al apparatus, such as storage batteries. Where several pieces of ajiparatus are thus controlled it is necessary to use an auxiliary relay, due to the delicacy of the sensitive relav, which cannot break currents in ex- cess of a few fractions of an ampere. An ordinary pony relav- of twenty ohms resistance is suitable for this purpose. If the arcing across its local circuit contacts is excessive a condenser con- nected in parallel with them will be found .idvantageous. The contacts should be set well apart and considerable tension put in (he back spring to (ounteract the leiidencv to stick. Re-

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