本刊1992年第4期《物理量的变化率》一文的第8例题是:如图1所示,四个线圈L_1、L_2、L_3和L_4。L_2、L_3连通,离得较远,线圈L_1和接线电阻不计,现让滑动变阻器R的触头P向下移动,使阻值均匀地变小,那么可以判定: The 8th example of the article “Change rate of physical quantity” in the 4th issue of 1992 is: As shown in Fig. 1, four coils L_1, L_2, L_3, and L_4. L_2, L_3 are connected, far away, the coil L_1 and the wiring resistance are not counted, and now the contact P of the sliding varistor R is moved downward, so that the resistance value becomes even smaller, then it can be determined: