  White dwarfs (WDs) can be tidally disrupted only by massive black holes
(MBHs) with masses less than $\sim10^5 M_\odot$. These tidal interactions feed
material to the MBH well above its Eddington limit, with the potential to
launch a relativistic jet. The corresponding beamed emission is a promising
signpost to an otherwise quiescent MBH of relatively low mass. We show that the
mass transfer history, and thus the lightcurve, are quite different when the
disruptive orbit is parabolic, eccentric, or circular. The mass lost each orbit
exponentiates in the eccentric-orbit case leading to the destruction of the WD
after several tens of orbits. We examine the stellar dynamics of clusters
surrounding MBHs to show that single-passage WD disruptions are substantially
more common than repeating encounters. The $10^{49}$ erg s$^{-1}$ peak
luminosity of these events makes them visible to cosmological distances. They
may be detectible at rates of as many as tens per year by instruments like
Swift. In fact, WD-disruption transients significantly outshine their
main-sequence star counterparts, and are the most likely tidal interaction to
be detected arising from MBHs with masses less than $10^5 M_\odot$. The
detection or non-detection of such WD-disruption transients by Swift is,
therefore, a powerful tool to constrain lower end of the MBH mass function. The
emerging class of ultra-long gamma ray bursts all have peak luminosities and
durations reminiscent of WD disruptions, offering a hint that WD-disruption
transients may already be present in existing datasets.
