We use 214410 hourly observations of the transpolar voltage, Φ_PC, from 25 years of observations by the SuperDARN radars, to confirm the central tenet of the Expanding-Contracting Polar Cap (ECPC) model of ionospheric convection that Φ_PC responds to both dayside and nightside reconnection voltages (Φ_D and Φ_N). We show Φ_PC increases at a fixed level of nightside auroral electrojet AL index with increasingly southward IMF (identifying the well-known effect of Φ_D on Φ_PC) but also with increasingly negative AL at a fixed southward IMF (identifying a distinct effect of Φ_N on Φ_PC ). We study the variation of Φ_PC with time elapsed Δt since the IMF last pointed southward and show that low/large values occur when -AL is small/large. We have to allow for the fact that at lower numbers of radar echoes, n_e , the matched potential re-analysis technique used to derive is influenced by the model used: this is done by a sensitivity study of the threshold of n_e required. We show that for any threshold Φ_PC falls to about 15kV for & Δt greater than about 15 hours giving an upper limit to the viscous-like voltage. It is shown that both Φ_PC and -AL _{increase with increased solar wind dynamic pressure psw , but not as much as the mid-latitude geomagnetic range index am. We conclude psw increases both ΦD and ΦN through increasing the magnetic shear across the relevant current sheet but has a bigger effect on mid-latitude geomagnetic activity indices via the additional energy stored in the tail lobes.}