Popular Science Monthly 



277 



cover and fill the containing jar to about 

 I in. from the top with a 25-per cent 

 solution of caustic jiotash in water. On 

 top of this solution pour a layer of heavy 

 paraflin oil about J/^ in. deep to exclude 

 the air and prevent creejiing. 



The battery is now ready for use and 

 needs no attention whatever till it is 

 exhausted and all the oxide of copper 

 reduced to metallic copper. This is of 

 great purity and is worth a good i)rice. 



The 25-per cent solution of caustic 

 potash is best made up in the following 

 way : 



First ascertain how much water the 

 container will hold when the filled copper 

 cylinder and zinc cylinder are in place. 

 Pour ofT about a third of the solution 

 and put in the amount of caustic potash 

 required. Stir very gently, since a drop 

 of that solution in your c>'e might cost 

 you your eyesight. 



A Clever Window-Display 



A MYSTERIOUS window-display 

 which attracts passersby and holds 

 their attention consists of an opal 

 ark-lamp globe practically full of water 

 in which an incadcscent lamp floats tip 

 up. At short inter\als the lamp lights 

 up brillianth' and at the same time 

 submerges itself in a very mysterious 

 way. After a few seconds it bobs up 

 and its light practicalK- fades out. This 

 cycle is continued indefinitely. The 

 only explanation for the odd nio%'ements 

 of the lamp seems to be some bared wire 

 ends projecting o\er the edge of the 

 globe, giving the idea that it is regulated 

 in its movements by wireless influence. 



The true explanation of the de\ice is 

 as follows: 



A field coil from an old dismantled 

 motor was placed in a box and within 

 was put an iron core. A small iron 

 pulley was actually used for this. An 

 iron bolt was put through the box cover 

 into the center of the core and the wires 

 for feeding the lamp run through the 

 cover alongside the bolt. The lamp was 

 connected in series with the coil; the 

 wires were soldered to the lamp base 

 and well protected by rubber tape. To 

 seal the bottom of the globe it was put 

 on a rubber sheet and a layer of sealing 

 compound filled in; an electric soldering 



iron was used to work the compound 

 well around the edge and about the bolt 

 head tt) make a water-tight seal. 



Fastened to the lamp base by a loop 

 o\er the tape was an iron wire with the 

 lower i)arl arranged in spiral form; this 

 was of just the right weight to keep the 

 lamp about half submerged when the 

 current was ofT. Connected into the 

 circuit was a Thermoblink flasher which 

 periodically cut the current in the lamp 

 and coil circuit down to a low value. 

 As the current was restored to full \alue, 



... rAV SHAPED BAQe 

 Wl^ WIRE TEfiMIHAiS 



60 WATT MAZOA 

 LAMP 



LUMINOUS ARC 

 OLOBB. 



TO fUkSHEa 



The mysterious electric lamp which glows" 



brilliantly one moment and the next 



fades away beneath the water 



the coil was energized and the iron spiral 

 with the lamp was pulled down toward 

 the bolt head, a sheet of tinfoil o\cr the 

 latter preventing actual magnetic con- 

 tact and sticking due to residual magnet- 

 ism when the current was again cut 

 down to its low value. 



The box on which the globe was placed 

 and the wires really leading to the coil 

 and lamp were covered by a cloth, leav- 

 ing very conspicuous, however, the wires 

 on the outside of the globe to the anten- 

 nalike ends. 



Lowering the Decrement 



THE present radio laws require that 

 every transmitting station shall 

 emit "sharply tuned" wa\es. The loga- 

 rithmic decrement must be less than 0.2. 

 The smaller this decrement the more 

 waves in each wa\e-train and the sharper 

 the tuning. With a decrement of 0.2, 

 which is really quite high, there are only 

 12>2 waves in a wave-train InMore the 

 amplitude has fallen ofT nine-tenths. 



