Fields induced by a beam and penetrated outside the beam pipe can be used for a beam diagnostic. Wires placed in longitudinal slots in the outside wall of the beam pipe can work as a beam pickup. This has very small beam-coupling impedance and avoids complications of having a feed-through. The signal can be reasonably high at low frequencies. We calculate the beam-coupling impedance due to a long longitudinal slot in the resistive wall and the signal induced in a wire placed in such a slot and shielded by a thin screen from the beam. We present a field waveform at the outer side of a beam pipe, obtained as a result of calculations and measurements. Such kind of diagnostic can be used in storage rings, synchrotron light sources, and free electron lasers, like LINAC coherent light source. Show Partial Abstract