If you are ever annoyed by dogs or cats that knock the cover off the garbage can in their search for canine and feline delicacies, you can give them the surprise of their lives with the aid of a spark coil upon the occasion of their next visit. Set the can upon a thick piece of perfectly dry wood and run a wire from the can to one secondary terminal of the spark coil. Connect the other secondary terminal to the ground or to a water or gas pipe. The next time you see a dog with his nose in the can, close the switch connected to the primary circuit. What happens will not hurt the dog, but he will travel for home as fast as four legs can carry him. Experiment 9—How to Make an Electric Spark Photograph Itself. This experiment must be conducted in a dark room with the aid of a photographer’s ruby lamp so that the photographic plates used will not become "light struck" and spoiled. Place an ordinary photographic plate on a piece of sheet metal with the coated or sensitive side up. Connect the piece of sheet metal to one of the secondary terminals of the spark coil by means of a wire. Then sift some dry starch powder, flour of sulphur or talcum powder through a thin piece of gauze on to the plate. Lead a sharp pointed wire from the other secondary terminal of the coil. Complete the primary circuit of the coil just long enough to make one single spark. Wipe the powder off the plate thoroughly and develop it in the usual manner. The negative will show a peculiar electric discharge looking very much like sea moss. No two photographs will be alike. The test results are obtained when only one or two sparks are allowed to pass. Arts and Science Series No. 9 Home-made Electrical Apparatus A Practical Handbook for Amateur Experimenters In Three Parts *Volume III* _Second Edition_ *BY* *A. M. Powell* PUBLISHED BY COLE & MORGAN, Inc. Publishers of the Arts and Science Series P.O. BOX 473 CITY HALL STATION NEW YORK, N. Y. Printed in U. S. A. Copyright 1918 by COLE & MORGAN, Inc. CHAPTER XII. HOW TO MAKE A DYNAMO-MOTOR