A silver-backed hand mirror on a lady’s dressing table looks very unlike an electrical instrument, yet a number of electrical experiments can be made with its help. A friend of the writer once described how he had used one in intercepting a wireless message. Perhaps he will be inducted to tell the story some day, but it is rather too complex foe this book.
There is, however, another experiment, one part of the which is very easily carried out, and the other part being more or less a matter of luck. The mirror must be a good one, and its surface so clean that, when you breathe on it gently, the mist will spread evenly over it without any suggestion of streaks. When the mist has entirely cleared away, polish it vigorously with a perfectly dry and slightly warm silk handkerchief. It is better not to press heavily, but to move the silk very rapidly. Hold the handle of the newly polished mirror in your left hand and press a clean half crown on the bright surface with one of your right fingers, taking care not to soil the mirror or to let the coin slide sideways. When you have held it in position for two or three seconds, turn the mirror over and let the coin drop off-again without any sliding.
Now breathe on the mirror again. The mist will form evenly at first but as it dies away the shape of the coin will appear, first the rim, and then the whole disc. When the glass is quite bright again, breathe on it once more. The image of the coin will still be there. That is the first part of the experiment. The second is a matter of chance. Sometimes, instead of a small disc, the pattern of the coin develops through the haze in considerable detail, but this only takes place occasionally; it is difficult to be certain why. Perhaps it depends on the length of time the coin is held in position, and no doubt the state of the atmosphere has something to do with it. Try it yourself.