A magnetic field using using Helmholtz coils is uniform in a small region and has a magnitude of $0.75\;T$ .In the same region a uniform electrostatic field is maintained in a direction normal to the common axis of the coils. A narrow beam of (single species) changed particles all accelerated through $15\;KV$ enters this region in a direction perpendicular to both the axis of the coils and the electrostatic field. If the beam remains undetected when the electro static field is $9 \times 10^{-5}V/m$ making a simple guess as to what the beam contains . Is the answer unique?
