Battery-powered fan You can pick one up for a buck at a dollar store. It should run off of two 1.5V AA batteries.

USB cable Also from a dollar store.

Resistor Ohm value TBD below. If you don’t already have the right value resistor, this ups the total materials cost to $3.

Multimeter

Soldering equipment

Wire cutter/stripper

7. Strip the USB cable. Strip the USB cable to expose the inside wires. There will be 4: power (red), ground (black), data 1 (green), and data 2 (white).

8. Strip and prepare the red and black wires. Fold or cut away the white and green wires. Strip the red and black ones, to expose about ½ inch of the copper threads inside. Twist the threads together for each wire, then tin them with your soldering iron.

9. Solder the red and black wires. Solder the

are on the same side as the motor and are usually red wire to the positive terminal in the battery identified with + and -, with the flat side positive compartment, and solder the black wire to the and the side with the spring negative. (You can negative terminal. ignore the other connector pair at the bottom of 10. Create an exit hole. Carefully, and in a well-the compartment. This just serves to connect the ventilated area, use the soldering iron to melt the two batteries in series.) plastic on the side of the battery compartment, 3. Measure resistance across the motor coil. creating an exit hole for your USB cable.

Use a multimeter to measure the resistance 11. Close up the fan. Run the USB cable through between the positive and negative terminals. the new hole and close the battery compartment This is the resistance across the motor coil. For and any other parts of the unit you’ve opened. example, you might get a reading of 10 ohms. 12. Let ’er rip. Plug the fan into your computer, 4. Determine the amperage. Now, some math. and enjoy the cool relaxing breeze as you admire Using V=IR (Ohm’s Law, voltage = current × your workmanship. resistance), figure out the motor’s amperage.

You have V and R; two 1.5V AA batteries produce Erica Sadun has written, co-written, and contributed to over 3V for V, and you just measured the resistance R. two dozen books about technology, particularly in the areas of So divide V by R to solve for I. For example, a programming, digital video, and digital photography. motor with 10 ohms resistance draws 0.3 amps.

Note that because USB can provide up to 0.5 amps, this project can only work if the motor’s load is over 6 ohms.

5. Calculate the additional resistance needed.

Next, determine the resistance you must add into the circuit in order to keep the amps about the same with a 4. 5 to 5V power source as it previously had with 3V of battery power. This is another Ohm’s Law calculation, but now you use

4. 5 for the V, your amperage from Step 4, solve for the resistance, and subtract the motor’s existing resistance. With our 10-ohm motor, this gives

4.5V = 0.3 amps × R ohms, or R = 15. With our original resistance of 10 ohms, this works out to

5 additional ohms. Find a resistor with this value.

6. Solder the resistor. Solder the resistor in series (not in parallel) between the motor and the positive battery terminal. To do this, cut the wire that connects the positive terminal to the motor, strip the new ends, and solder them to the resistor.

INTEREST OF MONEY

Dr. Price, in the second edition of his Observations on Reversionary Payments, says: “It is well known to what prodigious sums money improved for some time at compound interest will increase. A penny so improved from our Saviour’s birth, as to double itself every fourteen years — or, what is nearly the same, put out at five per cent compound interest at our Saviour’s birth — would by this time [1889] have increased to more money than could be contained in 150 millions of globes, each equal to the Earth in magnitude, and all solid gold. A shilling, put out at six per cent compound interest would, in the same time, have increased to a greater sum in gold than the whole solar system could hold, supposing it a sphere equal in diameter to the diameter of Saturn’s orbit.”

From Burroughs’ Encyclopaedia of Astounding Facts and Useful Information, 1889.

Download a copy from manybooks.net/titles/ burroughsb1409114091-8.html.