428 



Popular Science Monthly 



only a few thousandths of an ampere, 

 small flashlight batteries may be em- 

 ployed. The selenium cells should be 

 capable of carrying at least two or three 

 milli-amperes without heating. 



The next and probably the most deli- 

 cate step in the entire construction is 

 the ultra-sensitive relay that is placed in 

 circuit with each selenium cell. These 

 should operate reliably on a change in 

 current strength of as little as twenty- 

 five millionths of one ampere. 



The finest of polarized relays, such as 

 those devised 

 for use with 

 coherers in the 

 early stages of 

 wireless tele- 

 graphy, require 

 an operating 

 current of at 

 least five hun- 

 dred microam- 

 peres, or one- 

 half a milli-am- 

 pere ; the most 

 sensitive galvan- 

 ometer relay 

 with solid con- 

 tacts requires 

 about two hun- 

 dred microam- 

 peres. These 

 values are for 

 conditions of jar 

 and vibration 

 such as those 

 which naturally 

 exist in the elec- 

 trical dog. The 

 relays that I use are the most sensitive of 

 the pivoted, galvanometer type; but in- 

 stead of having two solid contacts of plat- 

 inum, one is made of platinum with a 

 needle point, and the other is a globule of 

 mercury, an arrangement which requires 

 a very small contact pressure for reliable 

 operation under vibration. 



A drop of light oil over the mercury 

 prevents oxidation. This contact, how- 

 ever, cannot break currents in excess of 

 a few milliamperes and should therefore 

 be used in conjunction with relays of the 

 telegraph type, which are capable of 

 handling the currents required in the 

 motor and solenoid circuits. Less sen- 

 sitive instruments cannot be used unless 



Diagram of 



the electrical 



connections 



the source of light be very powerful. 

 The sensitiveness of this arrangement is 

 so high that a dog can be operated with 

 ease from a distance of twenty feet with 

 a pocket flashlight. 



The pony relays indicated in the dia- 

 gram are ordinary telegraph relays of 

 twenty ohms resistance, provided with a 

 special pair of back contacts, which are 

 always closed when the relay is not en- 

 ergized. 



The motor is a ten-volt battery motor 

 of the largest size obtainable (about fif- 

 ty watts). Its source of power should 

 be a storage battery, which also supplies 

 the solenoids. In my apparatus this bat- 

 tery was composed of four four-volt, 

 thirty-ampere-hour cells. They shovild 

 be as small and as light in weight as 

 possible. 



The solenoids are approximately five 

 inches long and three inches in diameter, 

 with cores three-fourths of an inch in 

 diameter. Of the iron-clad type, they are 

 wound with number sixteen magnet 

 wire, and have cone-shaped pole faces, 

 the air gap being inside the coil near 

 the middle ; the stroke is about one-half 

 inch from the central position. Their 

 purpose is to turn the steering wheel. 



The core, which extends from one 

 solenoid to the other, is maintained in 

 the central position when both the sole- 

 noids are energized. 



The single rear wheel is mounted on 

 ball bearings in the horizontal plane to 

 facilitate turning by the steering mag- 

 nets. 



The reversing switches, by means of 

 which the dog can be made to back 

 away from the light, instead of being at- 

 tracted to it, are not shown in the dia- 

 grams as they would introduce an un- 

 necessary amount of complication. Their 

 purpose is to reverse the connections of 

 the two solenoids. 



The driving motor is connected to the 

 shaft of the two forward wheels through 

 a worm-wheel reduction, and a differ- 

 ential gear box, such as those on auto- 

 mobiles. 



The adjustment of the parts of the 

 dog is sometimes a rather difficult task, 

 particularly when other sources of illum- 

 ination besides the flashlight are en- 

 countered. If used in a room with win- 

 dows through which daylight passes it 



