A backing for an electrophotographic material has a surface resistance of less than 1010 ohms/square and is formed by coating a base material of higher surface resistance with a polymeric product obtained by the action of a protogenic or Lewis acid on an olefinic hydrocarbon. Suitable hydrocarbons include isoprene, piperylene, butylene, isobutylene, hexylene, cyclopentene, butadiene, dimethyl-butadiene and cyclopentadiene. A suitable mixture of olefins is the pre-benzene fraction obtained by distillation of the aromatic liquid product of a petroleum cracking and aromatization process. Also suitable is a mixture of mono- and diolefins obtained by distillation of the light oil obtained from coal tar. Cyclopentadiene is the only olefin used in specific examples. Suitable acidic materials include mono-chloroacetic acid, dichloroacetic, trichloroacetic acid, trifluoroacetic acid, picric acid, benzene sulphonic acid, diphenyl-phosphate, orthophosphonic acid, aluminium chloride, boron trifluoride, ferric chloride, boron fluoride etherate, stannic chloride, and aluminium bromide. The polymeric material which may be pre-formed or formed in situ, may be a surface coating only or may also fill the pores of the base material. Suitable base materials include cellulose web, paper, cardboard, cloth, nylon wood, plastic, leather, or parchment. Paper is the only base material used in specific examples. To modify further the surface conductance of the material various powders may be mixed with the polymeric material. Suitable powders include selenium, zinc, chromium, cadmium, aluminium, copper, iron, silver, nickel-aluminium alloy, selenium dioxide, zinc oxide, zinc sulphide, chromium trioxide, cadmium sulphide, alumina, cupric oxide, cupric sulphide, ferric oxide, ferric sulphide, graphite, animal charcoal, bentonite, soluble starch, cadmium oxide, and kieselguhr. Examples.-In one example an ester obtained by reacting cyclopentadiene and trichloroacetic acid (see Specification 819,826), is sprayed on to paper and left for polymerization to occur to give a deeply coloured polymer on both sides of the paper and filling the pores. In further examples, the ester mentioned above is sprayed on to filter papers covered with various powdered materials. The paper becomes coated with a mixture of the respective powdered material and polymer. In more examples, filter papers are respectively sprayed with aqueous solutions of trichloracetic acid, picric acid, benzene sulphonic acid, diphenyl phosphate, and orthophosphoric acid, and a further paper with a benzene solution of trifluoroacetic acid. After evaporation of the solvent each paper is sprayed with the ester mentioned above and left for polymerization to occur. In another example, a paper is sprayed with a chloroform solution of stannic chloride and with the ester mentioned above to give the rapid formation of a black shiny lacquer. In other examples, cyclopentadiene is added to papers impregnated with stannic chloride or with one of the acids in the examples described. Polymeric material is formed after the papers have been left for some days. That formed with trichloroacetic acid is described in Specification 711,791. Deeply coloured polymers are formed with diphenyl phosphate and trifluoroacetic acid. In a further example, a paper is sprayed with a chloroform solution of trichloroacetic acid and chloranil, and then with the ester mentioned above. The ester and chloranil form an insoluble polymer. Details are given of the separate preparation of the polymer. In more examples acid impregnated papers are soaked in a dilute solution of a polymer obtained by the reaction of cyclopentadiene and trichloroacetic acid. The preparation of this polymer is described. Paper may be impregnated with this polymer alone (applied as a solution in chloroform). Another paper is soaked with a colourless polymer obtained by the action of stannic chloride on cyclopentadiene. If trichloroacetic acid is added to the solution used a deeply coloured modification of the polymer is formed. In a final set of examples the polymer obtained from trichloroacetic acid and cyclopentadiene is reacted with maleic anhydride to give a further polymeric material. This is dissolved in aqueous sodium hydroxide to form a solution containing the sodium salt of an acid obtained by hydrolysis of the anhydride residues in the polymer. This solution is sprayed on to papers previously soaked with aqueous solutions of trichloroacetic acid, trifluoroacetic acid, benzene sulphonic acid, and diphenylphosphate respectively. The water is allowed to evaporate.ALSO:A backing for an electrophotographic material has a surface resistance of less than 1010 ohms/square and is formed by coating a base material of higher surface resistance with a polymeric product obtained by the action of a protogenic or Lewis acid on an olefinic hydrocarbon. Suitable hydrocarbons include isoprene, piperylene, butylene, isobutylene, hexylene, cyclopentene, butadiene, dimethyl-butadiene and cyclopentadiene. A suitable mixture of olefins is the pre-benzene fraction obtained by distillation of the aromatic liquid product of a petroleum cracking and aromatization process. Also suitable is a mixture of mono- and di-olefins obtained by distillation of the light oil obtained from coal tar. Cyclopentadiene is the only olefin used in specific examples. Suitable acidic materials include monochloroacetic acid, dichloroacetic, trichloroacetic acid, trifluoroacetic acid, picric acid, benzene sulphonic acid, diphenyl-phosphate, orthophosphonic acid, aluminium chloride, boron trifluoride, ferric chloride, boron fluoride etherate, stannic chloride, and aluminium bromide. The polymeric material which may be pre-formed or formed in situ, may be a surface coating only or may also fill the pores of the base material. Suitable base materials include cellulose web, paper, cardboard, cloth, nylon, wood, plastic, leather, or parchment. Paper is the only base material used in specific examples. To modify further the surface conductance of the material various powders may be mixed with the polymeric material. Suitable powders include selenium, zinc, chromium, cadmium, aluminium, copper, iron, silver, nickel-aluminium alloy, selenium dioxide, zinc oxide, zinc sulphide, chromium trioxide, cadmium sulphide, alumina, cupric oxide, cupric sulphide, ferric oxide, ferric sulphide, graphite, animal charcoal, bentonite, soluble starch, cadmium oxide, and kieselguhr. Examples. In one example an ester obtained by reacting cyclopentanediene and trichloroacetic acid (see Specification 819,826) is sprayed on to paper and left for polymerization to occur to give a deeply coloured polymer on both sides of the paper and filling the pores. In further examples the ester mentioned above is sprayed on to filter papers covered with various powdered materials. The paper becomes coated with a mixture of the respective powdered material and polymer. In more examples filter papers are respectively sprayed with aqueous solutions of trichloroacetic acid, picric acid, benzene sulphonic acid, diphenyl phosphate, and orthophosphonic acid, and a further paper with a benzene solution of trifluoroacetic acid. After evaporation of the solvent each paper is sprayed with the ester mentioned above and left for polymerization to occur. In another example a paper is sprayed with a chloroform solution of stannic chloride and with the ester mentioned above to give the rapid formation of a black shiny lacquer. In other examples cyclopentadiene is added to papers impregnated with stannic chloride or with one of the acids in the examples described. Polymeric material is formed after the papers have been left for some days. That formed with trichloroacetic acid is described in Specification 711,791. Deeply coloured polymers are formed with diphenyl phosphate and trifluoroacetic acid. In a further example a paper is sprayed with a chloroform solution of trichloroacetic acid and chloranil, and then with the ester mentioned above. The ester and chloranil form an insoluble polymer. Details are given of the separate preparation of the polymer. In more examples acid impregnated papers are soaked in a dilute solution of a polymer obtained by the reaction of cyclopentadiene and trichloroacetic acid. The preparation of this polymer is described. Paper may be impregnated with this polymer alone (applied as a solution in chloroform). Another paper is soaked with a colourless polymer obtained by the action of stannic chloride on cyclopentadiene. If trichloroacetic acid is added to the solution used a deeply coloured modification of the polymer is formed. In a final set of examples the polymer obtained from trichloroacetic acid and cyclopentadiene is reacted with maleic anhydride to give a further polymeric material. This is dissolved in aqueous sodium hydroxide to form a solution containing the sodium salt of an acid obtained by hydrolysis of the anhydride residues in the polymer. This solution is sprayed on to papers previously soaked with aqueous solutions of trichloroacetic acid, trifluoroacetic acid, benzene sulphonic acid, and diphenylphosphate respectively. The water is allowed to evaporate.