Abstract: The alternating bridge has got extensive applications in electronic industry, electric measurements of many non-electric quantities. However, the problem is that, in the application of the bridge, the electric power of the balance null indicator and that of the bridge must be connected to the ground at the same time, They each have earths and cannot be connected together. The problem can be effectively solved by using photocell and LED. A method which is used to solve the problem is reported.

Key words: alternating bridge, connecting problem, photocell, light insulates enlarging