fig1 a shows in cross section a flow - through heater 1 . this cross section is shown in perspective in fig1 b . the flow - through heater 1 comprises a channel 2 for guiding liquid to be heated , in particular water . a double walled cylinder 3 encases an induction coil 4 . the double walled cylinder 3 comprises an outer cylinder 5 of a ferromagnetic metal surrounding the induction coil 4 , and an inner pipe line 6 surrounded by the induction coil 4 . the inner pipe line 6 is in open connection and in line with a discharge line 7 . two annular end walls 8 , 9 close off the space 10 between the outer and inner walls 5 , 6 . the outer cylinder wall 5 surrounding the induction coil 4 forms an inner wall of an annular section 11 of the flow path channel 2 . the outer wall of the annular channel section 11 is formed by a casing 12 comprising a cylindrical wall 13 capped by a circular end wall 14 . the cylindrical wall 13 stands proud of the double walled cylinder 3 encasing the induction coil 4 , resulting in a space 15 between the circular end wall 14 on the one end and the annular end wall 9 of the double walled cylinder 3 on the other hand . at the other end , an annular end wall 16 closes off an annular space 17 between the double walled cylinder 3 and the cylindrical casing wall 13 . near the annular end wall 16 , a supply line 18 is joined to the cylindrical casing wall to form an open connection with the annular space 17 between the double walled cylinder 3 and the cylindrical casing wall 13 . the flow path for water to be heated is indicated in the drawings by the arrows . water flows from the supply line 18 via the annular channel formed by the annular space 17 between the double walled cylinder 3 and the cylindrical casing wall 13 , and further via the space 15 between the circular end wall 14 and the annular end wall 9 of the double walled cylinder 3 into the inner pipe line 6 of the double walled cylinder 3 and further into the discharge line 7 . at the inner side of the induction coil 4 , the generated magnetic flux heats the inner pipeline 6 , thus heating passing water . at the outer side of the coil 4 , the flux is shielded by the wall 5 . heat is generated in the wall 5 which is absorbed by the water passing the flow path . fig2 shows in perspective a cross section of a second embodiment of the heater 1 according to the invention . the heater 1 has a similar construction as the heater shown in fig1 a and 1b . parts common to both embodiments will be referred to by identical reference numerals . the embodiment of fig2 differs with the embodiment of fig1 a and b in that the outer surface 5 of the double walled cylinder 3 is provided with a radially extending spiral partition 19 . the spiral partition 19 defines a spiral flow path , serving to maximize heat transfer from the shield 5 to passing water . fig3 shows a further embodiment of the heater 1 according to the invention . again , parts common to both embodiments are referred to by identical reference numerals . in contrast to the embodiment of fig1 a and 1b and the embodiment of fig2 a double walled cylinder 3 is capped with a circular end wall 20 closing off the inner pipe line 6 . a discharge line 21 is joined to a cylindrical casing wall 13 opposite a supply line 18 . water flows from the supply line 18 to the discharge line 21 via an annular space 17 between the double walled cylinder 3 and the inner surface of the cylindrical casing wall 13 . fig4 shows a further embodiment of the heater 1 according to the invention . the heater 1 has a channel 2 . fluid to be heated enters the channel via a supply line 18 and exits the channel via a discharge line 21 . the fluid is guided by a second wall 13 and a coil encasing wall 55 around an induction coil 4 . before flowing through the discharge line 21 the fluid is guided through an inner pipeline 6 . the inner pipe line 6 is surrounded by the induction coil 4 . the second wall 13 has a ferromagnetic wall 5 . the coil encasing wall 55 may comprise ferromagnetic material , but this is not necessary as will be explained hereafter . ferromagnetic wall 5 serves as a shield protecting the environment against the impact of the electromagnetic flux . at the same time a part of the magnetic flux produced by the induction coil 4 may produce eddy currents in portions of the ferromagnetic wall 5 which portions comprise ferromagnetic material . in this embodiment the ferromagnetic wall 5 combines the electromagnetic shielding function with the possibility of heating the fluid which flows through the channel 2 of heater 1 . while the invention has been illustrated and described in detail in the drawings and foregoing description , such illustration and description are to be considered illustrative or exemplary and not restrictive ; the invention is not limited to the disclosed embodiments . other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention , from a study of the drawings , the disclosure , and the appended claims . devices , elements and components , known per se , have not been described in detail , as the skilled person is familiar with the matter . in the claims , the word “ comprising ” does not exclude other elements or steps , and the indefinite article “ a ” or “ an ” does not exclude a plurality . a single mechanism or other unit may fulfill the functions of several items recited in the claims . the mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage . any reference signs in the claims should not be construed as limiting the scope .