fig1 shows a first embodiment of the present invention illustrated in a somewhat schematic fashion . for a more detailed description of a general two - chamber detector reference is made to u . s . pat . no . 4 , 021 , 671 . in the detector , such as the one shown in this prior art patent , the electrodes 12 and 14 have a biasing potential applied thereto for establishing an ionization current in the chambers a and b . it is typical in a dual chamber construction to sense outputs from the center , common , or node electrode 16 also depicted in fig1 . in fig1 the electrode 14 may be of mesh construction , so that the chamber b is quite open . on the other hand , the chamber a is substantially closed except for a small hole to atmosphere . a hole is preferably also provided in the area of electrode 16 to permit communication to some extent between chambers a and b . the electrodes 12 , 14 and 16 , of course , are conductively separated and may be supported from a printed circuit board 20 such as is also shown in the prior u . s . patent referred to hereinabove . fig1 also shows the radioactive source 24 which may be in the form of a radioisotope pin or rod . this member 24 may also be a wire or ribbon and is supported from an adjusting screw 26 supported from the wall of the top chamber a . an insulating bushing ( not shown ) may also be associated with the structure coupled between the radioactive source 24 and the electrode 16 . this bushing and the arrangement of the source and adjusting member is substantially the same as shown in one embodiment disclosed in my co - pending application , ser . no . 959 , 102 , filed nov . 9 , 1978 . in accordance with the present invention there is associated with the ionization detector of fig1 a probe 30 having a manual actuating end or handle 32 extending through a wall of the chamber . the probe 30 is preferably provided with two separate conductive zones z1 and z2 . zone z1 may be wholly or at least partially conductive while the other zone z2 is less conductive than zone z1 or possibly non - conductive . it is preferred that the probe 30 make electrical contact with the electrode 12 . the handle end of the probe may be supported by being screwed into an internally threaded passage through the electrode 12 . the probe 30 may be of cylindrical shape with each of the zones z1 and z2 surrounding one - half of the cylinder . under normal operating conditions the probe 30 is in the position depicted in fig1 with the zone z1 facing the source 24 and with the probe 30 essentially functioning as a part of the electrode 12 . however , if the probe 30 is rotated 180 ° so that the left conductive zone or non - conductive zone z2 is presented or faces the ionization path then the ability of the probe to capture ions is reduced . this causes a lowering of the ionic current thus simulating ion capture by combustion products . once this calibration or testing has occurred then the probe may then be rotated back to the position indicated in fig1 . the simulation of ion capture can also be accomplished if the probe 30 is moved further away from the source 24 so that the probe is less likely to capture ions thus , likewise , reducing the ionic current . this could be accomplished in the embodiments of fig1 and 2 by moving or sliding the probe sideways or this could also be accomplished by partially or wholly withdrawing the probe from the chamber . of course , in order to provide adequate withdrawal the probe and chamber construction might have to be slightly different than that depicted in fig1 and 2 . fig2 shows a construction quite similar to the one shown in fig1 including the electrodes 12 , 14 and 16 , the printed circuit board 20 , chambers a and b , and the probe 30 . the probe 30 depicted in fig2 may be substantially identical to the one shown in fig1 . the chambers a and b may be , respectively , detection and reference chambers . the primary difference between the two embodiments of fig1 and 2 is that the embodiment of fig1 uses a single source whereas in fig2 there are shown radioactive sources 25 and 27 one being associated with each of the chambers a and b , respectively . the different modes of operation of the probe are substantially the same in fig2 as previously discussed with reference to fig1 . in the embodiments of fig1 and 2 it is preferred that the probe return essentially automatically to its normal position as depicted in fig1 and 2 . this can be accomplished quite easily by providing a torsion mechanism or a spring mechanism . such mechanisms are not specifically disclosed in fig1 and 2 but are well - known mechanisms that could be associated with the probe to return it to a particular predetermined position . fig3 a and 3b represent a further embodiment of the present invention which simulates the presence of particles of combustion by temporarily moving the radioactive source relative to the chamber electrodes . in fig3 a and 3b the electrodes are represented by the same reference characters as used previously . this embodiment also shows the adjusting screw 26 and radioactive rod source 24 supported from the adjusting screw 26 . in this embodiment the adjusting screw is not supported from the electrode 12 but it is instead supported from a movable plate 36 disposed at the top of the detector and having associated therewith a biasing spring 38 . as depicted in fig3 b the movable plate 36 has a flat side 37 against which the spring 38 may urge . in the embodiment of fig3 a and 3b the opening 17 for accommodating the source 24 may be larger than in fig1 and 2 so as to accommodate this lateral movement of the source 24 . also , a manual knob may be secured to the movable plate 36 for moving the plate against the bias of spring 38 to a calibrating position to simulate particles of combustion . the spring 38 then returns the movable plate 36 to a normal position when the knob ( not shown ) is released . fig4 and 5 show still another embodiment of the present invention . the embodiment of fig4 employs a single radioactive source in the form of a rod or pin source 24 supported from the adjusting screw 26 which is in turn threadly supported with the top section of electrode 12 . the chamber construction shown in fig4 including chambers a and b may be of substantially cylindrical shape and there is provided at the top thereof a tensioning disc or possibly only a strip 46 supported between the sidewalls of chamber a . a channel member may run about the inner surface of chamber a for accommodating the disc 46 . in fig4 the disc is shown in - solid in its upper position and in - dotted in a lower position . the disc 46 may be moved between these positions by means of the plunger 48 extending eternally at one end from the chamber a . the disc 46 normally lies substantially flat against the electrode 12 in the position shown in - solid in fig4 . the disc 46 is preferably conductive and may be perforated or actually of any other desirable shape which may depend upon the particular shape of the chamber a . upon operation of the plunger 48 the disc or strip 46 is tensioned to the position shown in - dotted in fig4 . this changes the geometry or volume of the chamber a thus altering the ionization current to simulate particles of combustion within the detection chamber a . the disc 46 is preferably made of a relatively thin metal that is quite readily deflectable . without the use of any separate spring mechanism the disc 46 may be in either position as a rest position requiring movement by means of the plunger 48 to move it to its opposite position . if the chamber geometry is such that it is desired to increased the electrode spacing or effective volume , then the normal position may be the position shown in - dotted while the plunger would then be used for calibration and test purposes to move the disc to the position shown in - solid in fig4 . the tensioning disc 46 may also be designed and preferably is designed so that the disc 46 has only one rest position and when it is tensioned to the opposite position and the plunger is released , the disc will automatically return to its original desired position for normal operation . fig5 shows a construction substantially the same as depicted previously in fig2 including two radioactive sources 25 and 27 associated , respectively , with the chambers a and b . fig5 also shows the tensioning disc 46 which may be of the same type discussed previously with regard to fig4 . the tensioning disc 46 in both fig4 and 5 is , of course , open at its center so that it can clear the radioactive source . also , in fig4 and 5 the disc 46 is shown in chamber a . it could also be disposed in chamber b or it can be applied in many other manners . for example , fig6 shows another dual chamber construction similar to the one depicted in fig4 including the electrodes 12 , 14 and 16 , printed circuit board 20 , adjusting screw 26 and pin radioactive source 24 . instead of the disc being provided at the top of chamber a there is provided a disc 46a having associated therewith a plunger 48a for operating the disc 46a . in the position shown in solid in fig6 the geometry of the chamber and its volume is at a maximum while the plunger 48a may operate the disc 46 to a position shown in dotted on a temporary basis to change the geometry of the chamber and thereby simulate particles of combustion . in fig4 - 6 , the deflectable member may be , as an alternative to the disc , a section of a disc or a strip . fig7 a and 7b show a construction substantially the same as previously depicted in fig5 employing a dual chamber arrangement including chambers a and b and also employing two sources 25 and 27 . the source 25 is preferably supported from the electrode 12 and may be insulatedly supported therefrom . similarly , the source 27 may be insulatedly supported from the electrode 16 . to provide the calibration in accordance with the present invention there is also provided a strip 52 which may be constructed of either a conductive or non - conductive material . the strip 52 is suitably supported at the top of chamber a and may be supported from the top section of electrode 12 . the strip 52 has near its center a hole 54 for accommodating the source 55 and a hinge pin 56 for hinging the movable strip 58 from the fixed strip 52 . the movable strip 58 has associated therewith a plunger 60 and a wing member 62 . in fig7 a the wing member 52 is shown in a vertical position in solid and is also shown in dotted in a horizontal position . the hinge arrangement is preferably spring loaded such as by having a spring associated with the hinge pin 56 . this spring biases the hinge member 58 to the position shown in solid in fig7 a . this is the permanent position of the hinge member and the position that it assumes when calibration is not occurring . when one wishes to calibrate the device , particles of combustion are simulated by urging the plunger 60 downwardly to temporarily move the entire hinge member 58 to the position shown in dotted in fig7 a . in this position the ionization source 25 is collimated . once the calibration procedure is over then the plunger 60 can be released and the hinge member returns to its initial rest position . this collimation of the output from the radioactive source is also provided in the embodiment of fig4 and 5 wherein the disc 46 has wings 47 extending downwardly from the center aperture of the disc 46 . this arrangement collimates ( attenuates ) radiation into the detection chamber in the second temporary position shown in dotted in fig4 and 5 . this attenuation or collimation effectively reduces the ionization current to simulate particles of combustion . similarly , the movement of the hinge member 58 in fig7 a and 7b also collimates radiation into the chamber in particular in the second temporary position shown in dotted in fig7 a and 7b thus effectively reducing the ionization current to simulate particles of combustion . herein the calibration means has been described in association with one of the chambers . however , in association with the invention it may be utilized with the other chamber , such as chamber b in fig1 and 2 .