When an electron is accelerated in a cyclotron, very soon it acquries a very high velocity. Due to it, its mass increases with velocity according to relation `m=m_0//sqrt((1-v^2//c^2))`. Therefore the time taken by electron to describe semicircular path inside the dee of a cyclotron, `t=pim//Bq`, also increases with the increases of m. Due to it, the electron does not arrive in the gap between the two dees exactly at the instant, the polarity of the two dees is reversed. As a result of it, the electron goes out of steps with the oscillating electric field and hence can not be accelerated by cyclotron.
