Popular Science Monthly 



427 



CONDENj 



will the dog go. By reversing a switch 

 on the outside of the box, the dog can 

 be made to back away from the light. 

 Illuminating both cells equally causes 

 the dog to move in a straight line. 



The electrical dog will 

 never become a common 

 household toy. It has taken 

 years of scientific study and 

 endeavor to perfect, and it 

 requires ripe technical knowl- 

 edge to understand clearly. 

 However, for the benefit of 

 the reader who possesses 

 more than an average amount 

 of scientific and technical 

 knowledge, a detailed descrip- 

 tion of the electrial dog is 

 given in the following lines : 



The mechanism involved in 

 the successful performance of 

 the electrical dog 

 is so complicated 

 and delicate in its 

 nature that it is 

 doubtful if many 

 experimenters will 

 care to attempt its 

 construction. Few 

 dimensions are 

 given, because the 

 materials naturally 

 convenient to the 

 builder have an 

 important bearing 

 upon even the 

 most detailed parts 

 of the apparatus. 

 The dimensions, 

 together with the 

 construction in 

 general, are largely 

 a matter to be de- 

 termined by the 

 builder's individual 

 ingenuity. T h e 

 general construc- 

 tion details sup- 

 plied here were 

 embodied in the 

 electrical dog, or 

 orientation mech- 

 anism, that Mr. 

 John Hays Ham- 

 mond, Jr., and I 

 constructed, a n d 

 which I have em- 



solenoids 



STEERING WHEEL 

 Diagram showing the electrical ap- 

 paratus used in the construction of the 

 Hammond Meissner Orientation Mech- 

 anism, or Electric Dog. Rays of light 

 striking the selenium cells cause the 

 motor and steering magnets to be oper- 

 ated. The light in the position here shown 

 causes the dog to go in a straight line 



ployed in lectures and demonstrations 

 before various engineering societies and 

 gatherings of all kinds. 



Beginning outwardly, the electrical 

 dog has these three dimensions : Length, 

 three feet ; height, one foot ; 

 width, one and one-half feet. 

 A small shelf projects from 

 the bottom of the box to- 

 wards the front. . This is 

 sawed or whittled almost to a 

 point, and a metal plate erected 

 extending four or five inches 

 outwards from a line drawn 

 exactly between the lenses. 

 The plate is there to prevent 

 light from going into one 

 lens when it is intended for 

 the other. 



The selenium cells should 

 be selected with great care, 

 and will cost from five dol- 

 lars apiece, up- 

 wards. The cells 

 are of as low a re- 

 sistance a s pos- 

 sible, this resist- 

 ance being at the 

 same time consist- 

 ent with a high re- 

 sistance ratio be- 

 tween light and 

 darkness. Putting 

 this thought into 

 concrete figures, 

 cells with a resist- 

 ance of from one 

 thousand to one 

 hundred thousand 

 o h m s normal or 

 ' 'dark' ' resistance 

 are the best. The 

 resistance of the 

 cell in the dark 

 should be at least 

 three times as 

 great as its resist- 

 ance in sunlight. I 

 have used cells of 

 sixty thousand 

 o h m s resistance, 

 and they gave 

 good results with 

 batteries of fifteen 

 or twenty dry 

 cells. Since the 

 current amounts to 



PIVOT 



