  We present RXTE observations of the soft gamma--ray repeater SGR 1900+14
taken September 4-18, 1996, nearly 2 years before the 1998 active period of the
source. The pulsar period (P) of 5.1558199 +/- 0.0000029 s and period
derivative (Pdot) of (6.0 +/- 1.0) X 10^-11 s/s measured during the 2-week
observation are consistent with the mean Pdot of (6.126 +/- 0.006) X 10^-11 s/s
over the time up to the commencement of the active period. This Pdot is less
than half that of (12.77 +/- 0.01) X 10^-11 s/s observed during and after the
active period. If magnetic dipole radiation were the primary cause of the
pulsar spindown, the implied pulsar magnetic field would exceed the critical
field of 4.4 X 10^13 G by more than an order of magnitude, and such field
estimates for this and other SGRs have been offered as evidence that the SGRs
are magnetars, in which the neutron star magnetic energy exceeds the rotational
energy. The observed doubling of Pdot, however, would suggest that the pulsar
magnetic field energy increased by more than 100% as the source entered an
active phase, which seems very hard to reconcile with models in which the SGR
bursts are powered by the release of magnetic energy. Because of this, we
suggest that the spindown of SGR pulsars is not driven by magnetic dipole
radiation, but by some other process, most likely a relativistic wind. The
Pdot, therefore, does not provide a measure of the pulsar magnetic field
strength, nor evidence for a magnetar.
