A common design of magnetometers for measuring rock magnetism is based on a circular double coil which acts as a detector of magnetic flux. If the linear dimensions of the coil and the distance from coil to specimen are expressed as multiples of the inner radius of the coil, a simplified expression for the electrical output of the coil results. It is then possible to construct a single family of curves applicable to the calculation of the electrical output of double coils of all sizes and proportions. Such a family of curves is presented. There are optimum proportions for which double coils reach maximum sensitivity. These proportions lead to wider and thicker winding sections for a given inner radius than usually used in double coils, but not much so. In fact, a coil of optimum proportions can reach the same sensitivity as a coil of lower proportions and have fewer turns, larger wire, and lower resistance. Sensitivity drops off rapidly for proportions lower than optimum, but drops off slowly for proportions larger than optimum. Thus, coils of small proportions should be avoided if possible. Sensitivity is best when the specimen is placed at the center of a coil of optimum proportions and when the inner circumference of the coil encloses the specimen as tightly as possible. If the inner radius of the coil is enlarged (with optimum proportions still maintained) to accommodate a larger specimen, sensitivity is further increased. Attention to all of these factors can lead to a sensitivity of from 30 to 45 times as large as the sensitivity that would otherwise result. Tables of maximum sensitivities and minimum signal for coils of various sizes and proportions are given as an aid in the design of coils.