The fundamental formula S_A/N = 5 ・ 10^<-7>π^2L^2/{In (2L/D) - 1}is reappraised to construct the most appropriate search-coil sensor for each of the three ways; routine observation at permanent stations, temporally observation at field stations, and space observations at spacecrafts. Calculation of the sensitivity and the coil weight derives an important conclusion that a direct winding around a slender core minimizes the net weight for a reasonable sensitivity. The most appropriate sensors according to our calculation are 1 = 2000, d = 20, N=10^5 for the permanent stations, 1 = 1000, d = 20, N =10^5 for the field stations, 1 = 400, d = 5, N = 2・10^4 for the spacecrafts, respectively, where 1 and d are the length (mm) and diameter (mm) of the core, respectively, and N is the number of the coil winding. A new idea to connect the search-coil sensors is proposed. Another new idea proposed herewith is to filter out the output signals from a high-sensitive ring-core mangetometer.