The uniform electric field approximation of parallel plates with guard rings is well recognized. However, near corners a high electric field exists in some circumstances, and it can give rise to electron or ion emission. A theoretical study of this field intensity, for example, shows that for a 10-mm electrode separation, 2-mm gap between an electrode and a guard ring, and a 0.005-mm corner radius, the electric field is as high as 5.4 times the uniform field found between infinite plane electrodes.