the present invention will be explained below with reference to the embodiments shown in the drawings . a perspective view of a current - collecting brush according to an embodiment of the present invention is shown in fig3 . in fig3 reference numeral 11 designates a brush . a multiplicity of brushes are arranged fixedly around the rotor . numerals 12a , 12b designate pigtails with an end secured to the brush 1 , numeral 13 a terminal having the other end of the pigtails 12a , 12b shown to be in the form of a pair of spiral conductors connected in parallel and fixedly secured thereto , and numeral 14 a connecting terminal on the terminal 13 for connection with a connector 4a of a support pillar 4 shown in fig1 . the pigtail 12a is formed in a clockwise spiral coil , and the pigtail 12b in a counterclockwise spiral coil . the inventors discovered while studying the spark generated between the slip ring , etc . and the brush that the pigtails of the collecting brush are greatly involved in the spark . specifically , the pigtails have a considerable length to meet the requirements for mounting on the brush holding case 3 from thereunder and the wear of the brush in operation . with the brush mounted on the brush - holding case , therefore , the pigtails are in a lengthy loose form . the pigtails thus have a considerable inductance , and the electromagnetic energy stored in the inductance is discharged the moment the brush comes off from the slip ring or the like , thereby increasing the spark generated . fig2 is a diagram showing an equivalent circuit for explaining a mechanism by which a spark is generated by the inductance of the pigtails . in fig2 numerals 9 and 10 designate a slip ring and a bus ring similar to those designated by like numerals in fig1 and numeral 15 a lead wire for connecting a power supply and the bus ring 10 . reference characters b 1 to b n designate a plurality of current collecting brushes including pigtails and brush units , characters r 1 to r n resistors therefor , l 1 to l n inductances thereof , and characters i 1 to i n currents flowing in the collecting brush units b 1 to b n respectively . character f designates a field coil of a rotary electric machine , character r f a resistor therefor , and character l f an inductance thereof . characters s 1 to s n designate switches for indicating the contacting and separation between the brush 1 and the slip ring 9 of the collecting brush units b 1 to b n . assuming that e is the voltage applied to a brush , e a the spark - generating voltage , r the pigtail resistance , l the pigtail inductance , i the pigtail current , w the arc energy , and t a the arc duration , while ignoring the resistance and inductance of the brushes themselves . under the condition where a spark is generated , the equations described below are established . ## equ1 ## assuming i 0 to be e / r and n to be e a / e , the equation ( 1 ) is substituted into the equation ( 2 ). then , ## equ2 ## as seen from the equation ( 3 ), the magnitude of the arc energy w is proportional to the magnitude of the inductance of the pigtails . it is therefore possible to dampen the spark by reducing the inductance of the pigtails . in the embodiment shown in fig3 the pigtails 12a and 12b are formed in oppositely spiralled coils in mutually alternate turns , so that the inductance thereof is very small thereby to reduce the spark generation effectively . the pigtails that are otherwise loose as in the prior art are extendable in spirally coiled form , with the result that the looseness of the entire pigtails is eliminated without lengthening the same as compared with the conventional pigtails . the foregoing description is made with reference to a case using two pigtails . alternately , three or more pigtails may be used . in this case , if an even number of pigtails are involved , they may be coupled in a plurality of pairs of oppositely spiralled coils in the manner shown at 12 in fig4 or groups of oppositely spiralled pigtails each including the same number of pigtails may be combined in one whole coil form in the manner shown in fig5 . if an odd number of pigtails are used , on the other hand , one of the pigtails is left as in the conventional manner as indicated by dotted lines in fig4 and 5 and the remaining pigtails are divided into groups each including the same number of opposedly spiralled pigtails into a coil form . in the latter case , the current flowing in the one separate pigtail left in the conventional manner is equivalent to only one of the divisions of all the pigtails , and therefore the inductance thereof is sufficiently small . even in the case where there are a plurality of pigtails not grouped in oppositely spiralled coils , the advantages of the present invention are maintained to the extent that the pigtails are formed in oppositely spiralled coils .