The device for measuring the current or emf is connected within the circuit of the thermocouple. It measures the amount of emf flowing through the circuit due to the two junctions of the two dissimilar metals maintained at different temperatures. In figure 2 the two junctions of the thermocouple and the device used for measurement of emf (potentiometer) are shown.
I think that this is what advertisers are shooting for when they pay the big price to get their ads up there. Regular billboards are just as effective as ever, even though there are now digital billboards being made that can have multiple ads on them and switch between them every few seconds.
To illustrate this, the cost of advertising on one billboard for a month is anything from Â£500 to Â£1,500 per month. Costs do fall substantially on a per billboard basis for larger campaigns but the overall cost means that only larger brands can afford this form of advertising.