In the red variety of Cucurbita pepo these crystals may consist of rods, thin plates, flat ribbons or spirals.
For experiments with long thin rods or wires it has an advantage over the other arrangements in that the position of the poles need not be known with great accuracy, a small upward or downward displacement having little effect upon the magnetometer deflection.
A man-engine consists of two heavy wooden rods (like the rods of a Cornish pumping plant), placed parallel and close to each other in a special shaft compartment, and suspended at the surface from a pair of massive walking beams (or " bobs ").
The rood varies locally but is generally taken as = 40 square rods,.
If, however, the permeability of the test rod differs from that of the standard, the number of lines of induction flowing in opposite directions through the two rods will differ, and the excess will flow from one yoke to the other, partly through the air, and partly along the path provided by the bent bars, deflecting the compass needle.