application of a suitable electric field conducting channels 233 form at the positions of maximum field enhancement electrons 236 are then emitted into the medium 237 with reference to fig3 structures of the kind illustrated in fig2 a may be produced by a flow coating process e g spin coating where a fluid medium 302 contains an insulating material and conducting or semi conducting particles 303 that due to their natural properties or surface coatings sometimes temporary do not wet the solution or dispersion containing the insulator and are exposed 304 as part of the coating process to form the desired structures 305 table coating may be employed using for example equipment such as that manufactured by chungai ro co ltd of japan examples of suitable insulating materials are glasses glass ceramics polysiloxane and similar spin on glass materials heated to reduce the organic content or form inorganic end products such as silica ceramics oxide ceramics oxides nitrides borides diamond polymers or resins examples of suitable particles are metals and other conductors semiconductors graphite silicon carbide tantalum carbide hafnium carbide zirconium carbide boron carbide titanium diboride titanium carbide titanium carbonitride the magneli sub oxides of titanium semi conducting silicon iii v compounds and ii vi compounds one suitable dispersion can be formulated from a mixture of a spin on glass material and particles said particles may be pre treated.