fig1 is a side view of a conventional slide bead coating device . a web 2 is moved while being laid over a break - up roll 1 and a coating liquid 4 supplied from a slide hopper type coating head 3 is coated on the web 2 while forming a bead 5 . a negative pressure chamber 6 is connected to a suction blower to maintain the pressure behind the bead 5 pg , 4 negative . fig2 is an enlarged side view of the bead . the internal pressure of the bead can be considered to be substantially uniform . therefore , on the upper and lower surfaces of the bead , p b = bead back pressure ( the pressure in the negative pressure chamber ), r 0 = the radius of curvature of the upper surface of the bead ( the sign being positive when the upper surface is downwardly convex ), and r b = the radius of curvature of the lower surface of the bead ( the sign being positive when the lower surface is downwardly curves ). thus , the effectiveness of the aforementioned u . s . pat . no . 2 , 681 , 294 can be theoretically ascertained . in general , as the degree of vacuum in the negative pressure 6 in fig1 is increased , the stability of the bead is improved and therefore a thin layer of coating liquid can be readily formed on the web at high speed . however , if the degree of vacuum is increased to a value higher than a certain value , then the bead may be broken by being pulled downwardly . accordingly , the most suitable degree of vacuum in the chamber 6 is , in general , obtained analytically from the results of experiments . as a result of various experiments , the inventors have found that breakage of the bead begins at the sides thereof ; that is , the bead is cut while shrinking widthwise . it is well known in the art that if the thickness of the side portions of the bead is increased , the stability of the bead may be improved . however , this method is not practical , because the amount of loss in the side portions of the manufactured web is increased . also , the drying capacity must be increased to the extent that the heavily coated portion can be sufficiently dried . in this invention , the atmospheric pressure for the middle portion of the bead is higher than the negative pressures for both side portions of the bead , whereby the tendency for the side portions of the bead to shrink widthwise is reduced and the bead is stably maintained even in high - speed thin layer coating . fig3 is a perspective view , with parts cut away , showing one embodiment of a slide bead coating device according to the invention . in fig3 components designated by reference numerals 1 through 5 are the same as those of the conventional slide bead coating device as shown in fig1 . the specific feature of this embodiment resides in having one negative pressure chamber 6 employed to maintain the pressure behind the bead negative in the conventional device but , according to the invention the chamber 6 is divided into three chambers 6a , 6b , and 6c . the degree of vacuum in each of the two side chambers 6a and 6c is made higher than that in the central chamber 6b . accordingly , the pressures applied to the marginal portions of the bead are set to a value between the degree of vacuum in the negative pressure chambers 6a and 6c provided at both ends and atmospheric pressure . the value is freely adjustably according to the degrees of vacuum in the chambers 6a and 6c . the degree of vacuum in the central chamber 6b can be determined independently of those in the two end chambers 6a and 6c . therefore , damage of the bead due to the higher degree of vacuum in the central portion of the bead can be prevented . the tendency of shrinking the side portions of the bead can also be prevented , and the bead can be maintained stable . this invention is not limited to the above - described example . that is , the technique of reducing the pressures applied to the marginal portions of the bead can be variously modified . for instance , as shown in fig4 the suction may be carried out through slots to effectively reduce the pressures applied to the marginal portions of the bead . furthermore , the negative pressure chambers may be formed so that they surround the marginal portions of the bead ( not shown ). this invention is applicable not only to the above - described slide bead coating method but also any coating method in which formation of a bead is essential . as is apparent from the above description , according to the invention a bead is maintained stable in the bead coating method . the effects of the invention are significant as described below : ( 1 ) the degree of stability of a bead in a bead coating method is increased , and therefore the coating can be achieved more stably . ( 2 ) even if the coating speed is changed , the bead can be stabilized in response to the change of the coating speed , and therefore the coating speed can be increased . ( 3 ) even if the amount of coating is changed , the bead can be stabilized in response to the change of the amount of coating , and therefore the thickness of a coated layer can be made thinner . ( 4 ) the bead can be stabilized without increasing the thickness of the marginal portions of the bead , and therefore the drying speed can be substantially increased . the effects of the invention will be more concretely described with reference to the following comparative example : a single layer of photographic emulsion ( with a viscosity of 40 cp and a surface tension of 28 dyne / cm ) was coated on a web by using a slide bead coating device as shown in fig3 . the flow rate of the emulsion was 20 cc / cm . min . under various bead back pressures , first the coating was done at low rate , and then the coating rate was gradually and continuously increased . during this operation , the coating rate at which the bead is broken ( called as &# 34 ; a critical coating rate &# 34 ;) was detected . the results are as indicated in tables 1 and 2 . table 1______________________________________ ( with conventional device ) bead back critical coating amount ofpressure ( p . sub . b ) rate coating remarks______________________________________ - 40 mm / aq 90m / min . 22 . 2 cc / m . sup . 2 - 50 mm / ag 150m / min . 13 . 3 cc / m . sup . 2 - 55 mm / aq -- -- the bead was broken byor less the large back pressure______________________________________ table 2______________________________________ ( with device of the invention ) bead back bead backpressure pressure critical ( central ( both end coating amount ofportion ) portions ) rate coating remarks______________________________________ - 40 mm / aq - 60 mm / aq 120 m / min 16 . 7 cc / m . sup . 2 - 80 mm / aq 230 m / min 8 . 7 cc / m . sup . 2 - 50 mm / aq - 60 mm / aq 190 m / min 10 . 5 cc / m . sup . 2 - 80 mm / aq 270 m / min 7 . 4 cc / m . sup . 2 - 55 mm / aq -- -- -- bead was broken . ______________________________________ as is clear from the comparison of table 1 with table 2 , the coating rate in the coating method according to the invention , in which the negative bead back pressure is increased for the both end portions of the bead , is increased by 50 to 100 % of that in the conventional coating method , and accordingly the thickness of the applied layer is much thinner . it is apparent that modifications to this technique can be made without departing from the essential aspects of this invention .