In order to measure the effects of gross changes in the design of the pole and yoke for a large synchrocyclotron cheaply and conveniently, a model half‐magnet having a two‐inch diameter pole was constructed. From data on the fields produced by poles of various lengths using a fixed amount of copper in the coil, the pole length for which the cost of power and magnet steel was a minimum could be determined. Measurements of the efficiencies of poles of various shapes indicated that it would be possible to design a shimmed pole to have an efficiency of 70 to 75 percent at a field of 20,000 gauss. Finally data on over‐all magnet efficiency as a function of the ratio of the cross‐sectional areas of the yoke and pole permitted calculation of the ratio for which the cost of steel, copper, and power would be a minimum.