Gear Trains
To create large gear ratios, gears are often connected together in gear trains, as shown here:

The right-hand (purple) gear in the train is actually made in two parts, as shown above. A small gear and a larger gear are connected together, one on top of the other. Gear trains often consist of multiple gears in the train, as shown in the next two figures.

In the case above, the purple gear turns at a rate twice that of the blue gear. The green gear turns at twice the rate of the purple gear. The red gear turns at twice the rate as the green gear. The gear train shown below has a higher gear ratio:

In this train, the smaller gears are one-fifth the size of the larger gears. That means that if you connect the purple gear to a motor spinning at 100 revolutions per minute (rpm), the green gear will turn at a rate of 500 rpm and the red gear will turn at a rate of 2,500 rpm. In the same way, you could attach a 2,500-rpm motor to the red gear to get 100 rpm on the purple gear. If you can see inside your power meter and it's of the older style with five mechanical dials, you will see that the five dials are connected to one another through a gear train like this, with the gears having a ratio of 10:1. Because the dials are directly connected to one another, they spin in opposite directions (you will see that the numbers are reversed on dials next to one another).