J.J Thompson was responsible for cathod ray experiment to determine the presence of electrons. Cathod ray tubes are easily found around us example is the cathode ray television which was used before the coming of plasma TV. cathode rays usually deposit an electric charge this can be done experimentally by putting an electrometer at the opposite end from the cathode and anode. When the cathode rays hit the electrometer, it measured an electric charge. During the experiment, he found out that charges carried by the cathode are usually negative since they move towards the positive end and away from the negative end. Using electrostatic attraction law that like charges repel, unlike charges attract, he was able to deduce that cathode rays are negatively charged.
