In the year 1897 by a British physicist Joseph John Thomson through his vaccum tube experiment concluded that the atom was not indivisible as previously claimed. In his experiment he supplied a high voltage in a vaccum tube consists of a two metallic plates. The plate which connect with the positive terminal was called as “anode” whereas which connects with negative terminal as “cathode”. He observed that at the anode end current starts flowing even through the circuit is not complete which let him to think that their is something which flows between cathod to anode. In order to get to know the direction of flow he applied a photosensitive substance behind both the plates and observe lighting behind the anode plate which led him to conclude that the direction of flow is from cathode to anode and called them as cathod rays. Later he applied positive and negative plate along the sides of the tube he observe that the lighting shifts toward the positive plate. This shows that cathod rays are negatively charged and instead of rays they are actually particles. His experiment also shows that the negative particles were alike andsmaller then atoms. Thomson also concluded that the negative particles were couldn’t be the fundamental unit of atom because atoms are electrically neutral.
Based on his finding Thomson put forward his model on the structure of atom which is simillar to that of a christmas pudding. He states that electrons are arranged in a spherical positive sphere as like dry fruits were arranged in a christmas pudding. For better understanding you can think of a watermelon and consider watermelon as a positive sphere and its seeds inside the watermelon were the electrons. He also added that the negative and positive charges were equal in magnitude which makes the atom electrically neutral.