as shown in fig7 alternator has a front bracket 1 , a rear bracket 2 and a rotor shaft 3 with the rotor shaft 3 being rotatably supported by bearings 4 and 5 disposed in the front and rear brackets 1 and 2 . the rotor shaft 3 has one end which extends outwardly through the front bracket 1 , with a belt pulley 6 to which the power is transmitted from the automotive engine generally designated by the reference character e being fixed to the outwardly extending end of the rotor shaft 3 . the alternator also has an induction type rotary field core 7 fixed to the rotor shaft 3 , with the rotary field core 7 having a cylindrical core 8 , a claw - type magnetic poles 9 and a collar member 10 . the claw - type magnetic poles include a pair of magnet poles 9a and 9b coupled to each other through a non - magnetic fixing member . the poles 9 are fixed to the front end surface of the cylindrical core 8 . the collar member 10 is fixed to the rear end surface of the cylindrical core 8 . as shown in fig2 the cylindrical core 8 includes six slots 12 are radially and equidistantly formed in the front end surface 8a , with the slots 12 being located at positions corresponding to the portions intermediate the claw - type poles 9 , i . e ., bases of the claws . axial grooves 13 are formed in the inner peripheral surface of the cylindrical core 8 in communication with the slots 12 . as shown in fig3 the collar member 10 includes six radial slots 14 formed in the end surface 10a where the collar member 10 is bonded to the cylindrical core 8 . the collar member 10 has a boss 10b for receiving the rotor shaft 3 . axial grooves 15 are formed in the inner peripheral surface of the boss 10b in communication with the slots 14 . thus , the grooves 15 and 13 provide oil passages which extend along the outer periphery of the rotor shaft 3 . the slots 12 provide oil passages between the front end surface of the cylindrical core 8 and the inner side of the magnetic pole 9a , while the slots 14 provide oil passages between the rear end surface of the cylindrical core 8 and the collar member 10 . the bearing 5 is arranged around the boss 10b of the collar member 10 and rotatably supports one end of the rotary field core 7 and the rotor shaft 3 . a field yoke 16 with a field coil 17 wound thereon is interposed between the inner periphery of the claw - type poles 9 and the outer periphery of the cylindrical core 8 , with its one end fixed to the inner surface of the bracket 2 by screws 18 . a minute gap 19 formed between the inner periphery of the field yoke 16 and the outer periphery of the cylindrical core 8 communicates at its one end with slots 14 and at its other end with slots 12 . a small gap 20 also is formed between the front end surface of the field yoke 16 and the inner side of the claw - type pole 9a . the gap 20 forms a continuous oil passage together with the minute gap 19 and the slots 12 , with one open end being directed towards a stator coil 23 which will be described later . a minute gap 21 also is formed between the rear end surface of the field yoke 16 and the inner surface of the rear bracket 2 . the minute gap 21 communicates with the slots 14 with its one end opening directed towards the stator coil 23 . the stator coil 23 is wound on a stator core 22 which is stationed on the inner side of the brackets 1 and 2 in a manner surrounding the rotary field core 7 . an oil inlet 24 in the form of a nipple is provided on a portion of the rear bracket 2 near the bearing 5 , with the oil inlet 24 communicating at one end with an oil passage space 25 formed between the bearing 5 and the inner side of the rear bracket 2 . the oil passage space 25 communicates with the grooves 15 . the other end of the oil inlet 24 communicates with a lubricating oil storage portion ( oil reservoir ) of the automotive engine through a tube and an oil pump which are not shown . oil outlets in the form of nipples are secured to bottoms of the brackets 1 and 2 with the oil outlets 26 being in communication with the oil reservoir through tubes ( not shown ). the alternator has a full - wave rectifier 27 which is fixed to the outer surface of the bracket 2 include contact therewith , at a location corresponding to the aforementioned minute gap 21 . a voltage regulator 28 is fixed to a cover 29 , and an oil seal 30 at the bracket 7 . as the rotor shaft 3 is rotated by the power of the engine , the induction type rotary field core 7 rotates . as the field coil 17 is excited in this state , an ac output is generated in the stator cord 23 , with the ac output being rectified by the rectifier 27 and , after a voltage regulation through the voltage regulator 28 , supplied to a battery ( not shown ) and electric load ( not shown ). during the operation of the alternator , the engine lubricating oil is supplied by the oil pump ( not shown ) to the oil inlet 24 , with the oil then flowing in the direction of the arrows through the oil passage space 25 and the grooves in the rotary field core 7 . the oil then shunts into two portions : namely , a portion which is introduced into the grooves 13 and then the slots 12 and a portion which is introduced into the minute gap 19 through the slots 14 . these two portions then merge in each other as they reach the minute gap 20 . a portion of the oil reaching the slots 14 flows into the minute gap 21 . the oil passing through the oil passages is sprayed in the form of vapor or mist onto front and rear portions of the stator coil 23 by the centrifugal force generated as a result of rotation of the rotary field core 7 , whereby the stator coil 23 is cooled by the sprayed oil . meanwhile , a fraction of oil which could not reach the stator coil 23 and a fraction of the oil which has dripped from the stator coil 23 attach to the field coil 17 thereby cooling the same . the field coil 17 is cooled also indirectly by the oil which flows through the minute gap 19 formed between the inner peripheral surface of the field yoke 16 and the outer peripheral surface of the core 8 . the rectifier 27 also is cooled indirectly across the bracket wall by the oil which flows through the minute gap 21 . then , the oil flows along the inner surface of the bracket and other portions and then flows down into the bottom of the bracket by the force of gravity . the oil then returns to the lubricating oil reservoir through the oil outlet 26 . it will be seen that the oil serves also to lubricate the bearings 4 and 5 . in the illustrated embodiment , the oil is recirculated through the described circuit so that the heat - generating parts such as the stator coil 23 , the field coil 17 , the rectifier 27 and so forth are effectively cooled . in particular , the stator coil 23 and the field coil 17 are highly effectively cooled by virtue of the direct spray of the oil vapor thereto , whereby the output characteristic of the alternator can be remarkably improved . in addition , the cooling means can simply and easily be formed by slots , grooves and minute gaps formed in or on the rotary field core , so that the size and the weight of the alternator can be reduced . furthermore , since the described alternator can have a hermetic construction free from invasion by external foreign matters such as dust and water , it can suitably be used as alternators on vehicles such as automobiles , small vessels and so forth . although a preferred embodiment of the present invention has been described , it is to be understood that the described embodiment is only illustrative . for example , as shown in fig4 a rotor shaft 3a may be provided which includes grooves 13a , 15a , respectively corresponding to grooves 13 and 15 , formed in the surface of the rotor shaft 3a rather than on the stationary side of the alternator , and the cooling means constituted by the slots 12 , 14 , grooves 13 , 15 and minute gaps 19 , 20 and 21 can be applied not only alternators for use on vehicles but also to various other alternators . further changes and modifications may be imparted thereto without departing from the scope of the invention which is limited solely by the appended claims .