  The CALICE collaboration is studying the design of high performance
electromagnetic and hadronic calorimeters for future International Linear
Collider detectors. For the hadronic calorimeter, one option is a highly
granular sampling calorimeter with steel as absorber and scintillator layers as
active material. High granularity is obtained by segmenting the scintillator
into small tiles individually read out via silicon photo-multipliers (SiPM).
  A prototype has been built, consisting of thirty-eight sensitive layers,
segmented into about eight thousand channels. In 2007 the prototype was exposed
to positrons and hadrons using the CERN SPS beam, covering a wide range of beam
energies and incidence angles. The challenge of cell equalization and
calibration of such a large number of channels is best validated using
electromagnetic processes.
  The response of the prototype steel-scintillator calorimeter, including
linearity and uniformity, to electrons is investigated and described.
