In a tube filled with helium, some positive rays have a charge to mass ratio of 2.41 × 10⁷ C kg⁻¹…

Cathode rays are electrons: negative, the same in every gas, with $e/m_e = 1.759 \times 10^{11}\ \text{C kg}^{-1}$. Millikan found $e = 1.602 \times 10^{-19}\ \text{C}$, so $m_e = 9.109 \times 10^{-31}\ \text{kg}$. Canal rays are positive, with $e/m$ depending on the gas. The neutron has no charge. In a field $E$, a charge $q$ feels $F = qE$.