as an impedance element and a transformer primary winding where the transformer primary winding has transformer coupling to at least one secondary winding exhibits voltage division between the load devices a detector placed across either the impedance element or the transformer primary winding detects a waveform similar to that of the ac source scaled according to the division ratio between the two load devices and altered in phase by the reactive component of the impedance of each load device the circuit into which the secondary winding of the transformer is wired couples to the primary winding for example if the secondary is an open circuit the waveform in the primary circuit will cause the detector to sense a particular signal from which can be extracted a dc voltage the magnitude of which is proportional to the impedance ratio of the impedance element and the coupled transformer components placing a resistive or reactive load or a short circuit across the transformer secondary winding will change the primary circuit ac output level this primary circuit ac output level is detected conditioned and used as an input to other electronic circuits fig1 illustrates an embodiment of the present inventive apparatus for a detector primary circuit 10 a sine wave generator 12 drives an impedance 14 and the primary of a transformer 16 in the mode shown both a first switch 18 and a second switch 20 are open corresponding to an unplugged call cord 22 a voltmeter 24 which serves here as a generic detector detects a baseline ac.