the solder used to effect the connection to tab 58 the component 33 is a sleeve of insulating material it will be understood that the diameter of the probe 40 as shown in fig4 and 5 is approximately 3 8 34 permitting it to be installed in the same housing as the transmitter previously described with reference to fig2 and 3 all of the electrically connecting terminals or tabs between the modules are accommodated within recesses so as to avoid any projections or protuberances that would interfere with insertion in the housing such as the housing 13 in fig1 before proceeding with a description of a presently preferred method of fabricating the modules 41 and 42 it may be helpful to consider the circuit produced by assembling the modules which circuit is illustrated in fig2 as shown therein the terminals or tabs 43 and 45 are connected through a series arrangement of resistors 65 the magnetically actuatable reed switches are designated by the reference numeral 66 each of which is connected in series with a separate resistor 67 a conductive bus 68 interconnects the conductive members or tabs 44 and 46 one end of each of the resistors 67 is connected to the bus 68 as shown the free end of the reed switch 66 is connected to a junction 69 between adjacent resistors 65 with the exception of the lowermost switch 66 on the right as seen in fig2 which is connected to the junction 70 between the terminal or tab 43 and the adjacent resistor 65 the length of the network between the upper terminals 45 and 46 on the one hand and the lower terminals 43 and 44 on the other hand may be varied as desired.