You can easily see in the background how much space this type of power system took up. It is possible that this photo was taken during World War I. This factory was clearly more safety-aware than most; note the screens to protect the operator if the belts broke, and the fire extinguisher on the pole at the center.

The joints in the belts -- you can see one just above the foreground machine on its belt -- functioned as fuses.

The woman on the right is holding what is most likely the control of an overhead idler wheel that functioned as an on-off switch.

Imagine the noise and the smell!

This is one of my favorite pictures, both for the richness of background information and for the expressions on the faces of the two women (and that of the grumpy, skeptical supervisor behind them).