The famous electrician Franklin (B. Franklin, 1706-1790) once observed that cork ball in the metal cup was completely unaffected by the electric charge on the metal cup. He wrote a letter for Priestley and told his observations, expecting him to redo this experiment, and confirm this fact. In 1766, Priestley did the experiment, and he electrified the metal cavity vessel and found there was no electric charge on its inner surface, and obviously there was no acting force of metal vessel on electric charge inside. He immediately thought that this phenomenon was very similar to universal gravitation situation, namely the matter within the homogeneous material spherical shell will not be subject to the acting force of the shell material. Thus, he guessed that electric power and universal gravitation are applicable to the same rules, and acting force between two electric charges shall be inversely proportional to the square of the distance between them. This is a very important analogy speculation, but the speculation at that time did not attract the attention of scientists. At the same time, Priestley himself had no confidence in strictly proving this speculation, so this finding was shelved.