Measurements have been made of the effectiveness of the ring shaped neutron sources for the MERLIN reactor. The emission rate of one neutron source was measured absolutely at the National Physical Laboratory and the remaining sources were compared with this standard. A neutron to gamma ration of (0.7 ± 0.1) × 16⁶ n/s per curie of ¹²⁴Sb was obtained. The effectiveness of the sources was found by measuring the power generated in a sub-critical core. This result was in reasonable agreement with a calculation, based on two-group diffusion theory.