lit 



8 



Popiihir Science Mnnilihj 



iinisiblo, l)iit by no means silent drama 

 wliicli 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"\vere 

 |)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 



^^ 



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- 



