Page:Popular Science Monthly Volume 88.djvu/455

 Popular Science Monthly

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��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

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