Abstract: In the embodiments described in the specification, an ink jet system includes an ink jet head with an ink reservoir and a passage supplying ink from the reservoir to a plurality of pressure chambers from which ink drops are ejected through a corresponding plurality of orifices, along with a viscosity-detecting arrangement for detecting the viscosity of the ink supplied to the pressure chambers and a control system for controlling a heater so as to control the temperature of the ink in order to maintain viscosity at a desired level. In a particular embodiment, the ink-viscosity detector consists of four fluidic elements having different fluidic resistance and inertance values connected in a bridge circuit which is arranged to be balanced when the ink has the desired viscosity value.

Abstract: The transparent hot melt jet inks described in the specification include a coloring agent, a vehicle for the coloring agent including a thermoplastic nontransparent base material and a transparentizing agent containing a glycol ester of a C.sub.18-35 fatty acid, a glyceryl ester of a C.sub.18-36 fatty acid with metal soaps, a polyethylene wax having a molecular weight of 400 to 1200, a synthetic paraffin wax, or a linear alcohol with a molecular weight of 500 to 1000 or a mixture thereof, a microcrystalline or modified microcrystalline wax.