An introduction to the tin dioxide (SnO,) ceramic gas sensor and its electrical characterization is presented, followed by details of some methods of active material preparation. Problems of sensitivity, selectivity and their modification are then considered. The basic measurand is the conductance of the sensor, which means that the active material must always bridge a pair of electrodes; it must also

We have measured the frequency of the (171)Yb(+) 12.6 GHz M(F)=0-->0 ground state hyperfine "clock" transition in buffer gas-cooled ion clouds confined in two similar, but not identical, linear Paul traps. After correction for the known differences between the two ion traps, including significantly different second-order Doppler shifts, the frequencies agree within an uncertainty of less than 2 parts in 10(13). Our best value, based on an analytic model for the second-order Doppler shift, for the frequency of the clock transition of an isolated ion at zero temperature, velocity, electric field and magnetic field, is 12642812118.466+0.002 Hz.

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