If you are really to understand the Electric Show you should go twice. I advise going with this key alone first and note down all the applications of electro-magnets which you can find there. When you have done so I shall be glad to have your report. VI APPLICATIONS OF THE ELECTRO-MAGNET It became quite the rage now among the boys to find as many uses of electro-magnets as possible. These were reported and explained to the club and a list kept. This list included: 1. Dynamo. 2. Magneto. 3. Ammeter. 4. Wattmeter. 5. Motor. 6. Electric piano and organ players. 7. Electric door openers. Already noticed in the preceding pages, and the following: 8. _The Electric Spinner_ (Fig. 26).--A toy full of instruction. The standard is a steel magnet which produces a magnetic field. Inside of this is an electro-magnet which serves as an armature. Plainly visible on its shaft is a commutator to which the electric current from a dry cell is sent. This causes the armature to revolve and carry with it a series of colour disks which may be adjusted so as to show what tint or shade results from mixing colours in various proportions. [Illustration: Fig. 26] [Illustration: Fig. 27] [Illustration: Fig. 28] 9. _The Electric Engine_ (Fig. 27).--This toy, with one dry battery cell, develops power enough to run several other toy machines. The diagram in Fig. 28 will make its plan of operation plain. _B_ is the battery cell, _c_ the electro-magnets, _a_ an armature of iron. By a rod this armature is connected with a crank on the axle which carries the fly wheel _f_. Another crank, _d_, upon the same axle serves like a push button to close the electric circuit at the right instant. The wire _g_ from the battery cell encircles the electro-magnet _c_ and then is connected to the iron base of the toy. When the crank _d_ touches the conductor _e_, which is a spring, the electric current passes around the magnet, the magnet pulls the iron armature _a_, and this gives an impulse to the wheel _f_ whose momentum carries it around during that portion of the revolution when _d_ is separated from _e_ and _a_ is receding from the magnet. It is customary to say that the circuit is closed through the base of the machine, but this language requires interpretation. It means that a way is provided for the electric current to pass through the base. A person who is expert in language but not in electricity might expect us to say "the circuit is open through the base." [Illustration: Fig. 29]